At the moment I'd recommend either the iPhone 4s or Samsung Galaxy S2 if you're looking for phones powerful enough to run music apps with ease.
But if you don't care too much about brands and operating systems, you might want to investigate the music apps first and then decide.
Personally... I'm really digging Android on the Samsung Galaxy S2.
P00K wrote:Blah blah iphwn @ndroid blah use them both
Excellent point indeed, in the end I prefer making beats on my MV (sold my SP-404 yet again unfortunately.. simply didn't use it often enough).