Korg stuff is on sale right now actually.
I like the new Figure app a lot, especially for quickly getting ideas down. Shame you can't save multiple songs yet, but I hear it's coming.
Of all the synth apps, I like Animoog the best. Sounds great, and easy to program.
If you want a more DAW style app then it's a toss up between NanoStudio and Grageband for me these days. GB is easier to get basic ideas down, but as a result there's less flexibility when it comes to sound programming. But, you can always open it in Logic on the MAc and go from there too. NS is very configurable, but lately I feel that I spend too much time jump through screens trying to do sound design with it.