Nara or Osaka?

I have to opportunity to travel to kyoto, and i want spend a whole daytrip close to Kyoto!
The best recommented cities are Nara and Osaka i guess.
But there is only one day free to travel somewhere near, because I want to see Kyoto too.

In addition to that, my japanese friend said, i should go to Nara, but I tend to Osaka.

Which do you preferred for a trip???

I prefer Nara. But it entirely depends on personal interests and the context of your itinerary. They are two very different cities!

Nara:
http://www.japan-guide.com/e/e2165.html

Osaka:
http://www.japan-guide.com/e/e2157.html

As Nara is not so big, and all atractions close early, you can go in the morning to Nara and then, around lets say 15.00, you can take the Kintetsu train to Osaka Namba (40 minutes) and, at least take a taste of Osaka.. & then go back to Kyoto (from Osaka Station, last train around 23.30)

Nara: History, culture, relatively quiet when compared with larger cities, green spaces etc
Osaka: hustle and bustle, great food, shopping, and neon extravagance at night.
I also recommend an early start, Nara during the day to mid-afternoon, then off to Osaka for a taste of bright lights and fabulous food options in the evening around Dotombori/Namba.

Hi,
I'm Japanese and living in Tokyo.
If you are interested in history, I recommend Nara.
Because Nara is older than Kyoto.

I love Osaka, especially, foods and people.
But foreign tourists may feel same with Tokyo.

If the weather is fine, I recommend you to buy an Osaka Unlimited Pass (very Osaka name!) and visit many tourist spots including Floating Garden Observatory, HEP Five Ferris Wheel, Santa-Maria Cruise in bay area and more.

From some towers, you can see Osaka bay, Kobe city, Ikoma mountains (Nara is in the other side of the mountain, and you can say you also see Nara) and more importantly Awaji island, which is most important point that Osaka is not like Tokyo, but superior to it (sorry, this is one of the traditional joke about Osaka).

http://www.pia-kansai.ne.jp/osp/en/05_a.php

I second Nara in the moning, Osaka in the afternoon. It will be a long day but Osaka, especially Kita Umeda and Minaki Namba, are very lively at night.

How much time do you have?
Osaka is a big city. Party town and good eating, shopping etc. If you have already experienced a city...
Nara is old Japan. Oldest in history (Japan). So if you want to experience Nara and take in what is there and it's culture you should see Nara. There's few Ryo-kan with Japanese style ofuro right cross from Dear Park if you must stay.
A lot of people pass up Nara because it's sort of out of the way.
After sight seeing Kyoto most people thinks they've seen it all.

You can visit both cities in a day.
It takes 35 minutes from Kintetsu Nara station to Kintetsu Namba. (Kintetsu Nara station is located at close to Kofukuji temple).
You can enjoy Osaka's evening at Namba after enjoying Nara fully.