Yakushima or Kyushu

Currently I am stuck in a Dilemma of sorts.

I am travelling with 2 other people, and we have already booked a lodge to stay in for the nights of the 26-27th, On the 25th April we will be staying in Kagoshima so we can get the early Jetfoil out or Ferry.

28th we head back to Osaka from Yakushima (No idea when we will be leaving.)

My problem is I feel like I am missing out on Kyushu. Kagoshima would literally be only Place I would see.

Now I have heard from others Yakushima is a piece of Heaven on Earth if you like Hiking,

But lets stay I drop out on Yakushima, and stay in Kyushu for 2 nights. Where would you recommend I go and what should I see?
And where is best to stay to give me decent access to other locations by train. (So basically a city to act as a hub for me to get to other locations without travelling for more than 3 hours by train)

From Kagoshima, Ibusuki and Kirishima are worth considering, especially if you love onsens.
Ibusuki: http://www.japan-guide.com/community/scarreddragon/report-2291
Kirishima : http://www.japan-guide.com/e/e4628.html

I'd go back to Yakushima tomorrow given the chance (without the hiking - the place I stayed was fine on a cliff overlooking the Pacific), but from Kagoshima I would do the sand-baths, Chiran and Sakurajima. There are a lot of other destinations, but you can't do everything.