actually using windows is only way to deliver games for iphone with udk, udk doesnot work on mac(for now, i think editors will still be pc only but front-end may get to mac.). Did not read much in to it but i guess you still need licenses from apple etc. Documentation pretty much says that you only need MAC when you are submitting your app to the appstore, no word of MAC on development/deployment stage, only distribution.

For android, from udk point of view i am guessing it will be pretty much same as iphone - that is of course when udk gets android support.

Being develeoped 2D app for both iphone and android(same app on both devices), i would say develeopment complexity was pretty much the same, sometimes both platforms needed some small workarounds where other one did not, in different areas. But that was all 2d and was not a game.(Can't be more clear, still under the effects of NDA)