Sakurajima or Shiroyama?

Which place is better to visit for a day trip in Kagoshima: Sakurajima or Shiroyama? I want to know which one has the best scenery and nature. I also like onsens and the one on Sakurajima has poor reviews, but there's supposed to be a good one on Shiroyama (Satsuma no yu). I won't have a car. Will it be easy to access Satsuma no Yu or will I have to hike for hours? Please give me advice.

You can do both... Shiroyama is right in the middle of the city. The onsen there is open pretty late if I recall correctly. You could go to Sakurajima in the morning and Shiroyama in the afternoon, or vice versa.

I have done both places as a day trip before (with Senganen Garden included in the trip) and you can easily visit both locations.
Both locations are great to visit. Shiroyama (and Senganen Garden) offers great views of Sakurajima, and I'd say the gardens off the better of two views.
Whilst Sakurajima has a lot of offer. Great up close views of the volcano and any eruptions, I saw 3 whilst there. There are also many interesting spots such as the Nagisa Lava Trail, the free foot baths and the observation decks. And it should be obvious that you get amazing views of the volcano whilst in transit on the ferry.

The way I did my trip was Kagoshima Chuo Station - Shiroyama -Senganen Garden - Sakurajima - Kagoshima Chuo Station.

I'd recommend getting a unlimited bus pass for 600yen that you can then use on the city view buses.

You should also check the JMA website as the status of the volcano does change. Right now it is at Level 3, but only too recently the level was upped to Level 4. http://www.jma.go.jp/en/volcano/