in general, it depends on why you want an ipad/iphone or android. if you just want it for a cheap adjunct to your main laptop or PC, say, as a controller, then ipad/iphone is probably still king here.
however, the recent push ('ultrabooks') are basically notebook PCs with touchscreens that are in a similar form factor to tablets (usually with a keyboard attached, which often either folds under the screen, or detaches completely, for tablet-type use), and use special low-voltage versions of notebook chips. they aren't quite comparable to the normal-voltage versions in power, but they are probably more capable than standard chips of a version or two back (i haven't looked - see disclaimer above). and, they are truly expensive.
given that there aren't really any touchscreen interfaces native for any DAW i know of yet, probably not that helpful today. but soon, you might be able to run your studio and the touchOSC controller on the same laptop...
