Hiroshima vs 1 extra day in Kyoto area

It's my first time in Japan and I'm not sure how to prioritise what to see, classic problem I guess. I've only got 4 days to do both Kyoto and potentially Hiroshima.

I understand the Peace Memorial Museum is under renovation. The Main building is supposed to be closed in May when I'll be in Japan.

Is the Main building the main attraction at the museum?
Is it worth going to Hiroshima vs doing Kyoto at a more relaxed pace, maybe spend a day at a nice ryokan with an onsen?

To me, the highlight of Hiroshima was Miyajima; not so much the Peace Park (which I did visit). Only you know what your priorities are, so I won't suggest to skip the Peace Park.

It is possible to visit both Miyajima and Peace Park in one day, if must.

For the first time visit, I would recommend to see as many different places as the time/budget allows.

I would leave Hiroshima for next time & enjoy the more relaxed time in Kyoto. For a first visit there is far more there than you could possibly see in the time you are there. Or you could spend the day going to Fushimi Inari & Nara which is only a short rail journey away.

To me, the visit to Hiroshima was one of the highlights of my first trip to Japan. The Dome is breathtaking both on the day and night, and the Peace Park as a whole was (and still is) the most humbling experience in my trip. I spent two nights there and also visit Miyajima Island. Highly recommended.

Is the Main building the main attraction at the museum?

If you have strong interest in "the history", yes , the museum could be the main attraction. Otherwise, the Dom as well as Miyajima would be my strong recommendations for the day trip.

I love Hiroshima and will be visiting for the forth time in two months time . I just think that the museum is an essential part of understanding the place and that there is too much to see on a day trip hence my suggestion to leave it for next time. A one day trip to Miyajima relies too much on the weather & state of the tide to be a success for me.

This is a tough one and, as you said, always the problem as there is no end to the days you could spend in Kyoto, let alone the rest of Japan. But you are seeking some guidance; therefore:
Three days for Kyoto and nearby Nara (18 minutes away I think). Fushimi Inari is part of Kyoto. Beyond that, I'll leave you to figure out your itinerary within the riches of Kyoto area.
Don't skip Hiroshima. I have done Hiroshima and nearby Miyajima all in one day including shinkansen to and from Kobe three times now. The museum is informative but its the Peace Park and its monuments That will get to your heart. (Assuming a degree of belief in world peace and no nukes so apologies if I assume wrong.) Hiroshima is much more than a mournful monument to past horrors; the city is vibrant, very accessible, and quite used to gaijin, that's you and me, bub. You could spend a fantastic couple of hours working your way back towards the JR station along Hondori, munching on Okonomiyaki as you go. (The oysters from the waters off Hiroshima are the sweetest I've ever had, comparable to Ise-shima and far tastier even than in New Orleans.) When you're tired of city exploration, grab one of the streetcars back to the station and head down to Miyajima for the deer, the floating shrine, and a breath of sea air. The more of your day you can spend at Miyajima the better.
When you are ready for your second trip to Japan, and you WILL be; you can reflect back on your explorations around Kyoto and Hiroshima and have a better idea of what areas you want to further explore then. As for me, a return to Hiroshima is two trips away due to "priorities" but I cant wait to explore Miyajima Island and go see the beautiful and overlooked Shukkien Gardens. Have a great first trip and Yokoso o Nihon.

Hiroshima is one of my favourite cities in Japan (dare I say much more than Kyoto) and I think it would be a pity to miss it. However, it depends on how much interest you have on the bombing and history of the place; if you don�ft care too much for history then you might just visit some other place and find it more interesting.

I went last summer when the building was under renovation also, and even though I only got to see half the museum I still think it was worth it. I�fm sure they�fll move the most important and pertinent information into the other areas so the visitors can see it.

For what it�fs worth, you can still enjoy a ryokan even with three days in Kyoto; most people spend three days there anyway.

I've been there five years in a row, 6 if you count my trip in a month or so. I go there for personal reasons as well as using it at a base for trips nearby (Iwakuni, Miyajima, Kure, Onomichi, Fukuyama, Saijo, etc.) It's a lovely place. It has my favorite okonomiyaki-ya, where the owner knows me by name even after 14 months. It's large enough to get a city feeling and still retains a small town atmosphere in places. The Peace Park is a must-see, especially when the school kids are there on their field trip to Hiroshima/Miyajima. I can't go to Japan without going to Hiroshima.

"It has my favorite okonomiyaki-ya, where the owner knows me by name even after 14 months."

You have to give it up. Hiroshima-style okonomiyaki is one of the main reasons I'm visiting Hiroshima (right after Miyajima and the Peace Park) in March. What's it called? Where is it located?

Oh I was going to ask that as well for when I am there in early April :)

Nagata-ya, just off the bridge from Peace Park that is located by the Rest House. It's popular with tourists.

Thanks! I already had it starred on Google Maps!

Always good to get a recommendation. I have used this restaurant on several occasions :) https://www.google.co.uk/maps/@34.3926788,132.464212,3a,75y,15.62h,79....

@John B Digs
Nagata-ya, just off the bridge from Peace Park that is located by the Rest House. It's popular with tourists.

Just off the bridge from Peace Park, near the Atomic Bomb Dome, right across of 7-11. Right?

Dangit. It's closed on Tuesdays. I have Hiroshima/Miyajima scheduled on a Tuesday. Wednesday is also closed. I was going to switch days but guess I'll have to find elsewhere to eat.

4.5 stars on Yelp!

Okonomiyaki Nagataya in Hiroshima is reviewed on Yelp! 39 times, in English. That's very rare. (Yelp! is mostly an American review site that has some international reviews.)
39 reviews - that's a lot.

https://www.yelp.com/biz/%E3%81%8A%E5%A5%BD%E3%81%BF%E7%84%BC-%E9%95%B...

Yes, Nagata-san is very good at his job! The minute I walked in he said, "John!" and then when I sat down he asked me about my shodo-sensei friend who lives in Hiroshima. And when I told him I was visiting Onomichi the next day, when I returned to the restaurant a few days later, he asked, "how was Onomichi?". He really remembers! I like that very much.

It is unfortunate it is closed on Tuesdays. :-(