sorry i am the first ,and i am a old fashioned grumpy old man and as such i still prefer wires everywhere , so for me there is no question but a answer :USB just because i'm to lazy to stockpile batteries to keep the keyboard working
but at least for the magic mouse you get a inductive charger
http://geekbeat.tv/magiccharger/
if you get the magic mouse get that charger , if you dont want to search for batteries in the middle of work all the time as some of these magic mice eat batteries for lunch some even in a less then two weekly interval , dont know what apple was thinking when they did build a device like a mouse that needs battery changes so often , a bit counterproductive in apples going green department
just in comparison my microsoft arc mouse (old model with scrollwheel) from microsoft needs only a battery change every 4-6 month , my mouse is in constant use nearly 24/7 with only small rests and last time i changed the batteries was i think mid /end of october , so i guess in about the first week march the little window will come up to warn me about low battery and gives me roughly 2 weeks to prepare myself to get some charged ones
i mean 3 month should be a absolute minimum for batteries to last in such devices , better would still be if you could have the option of making them usb , which should not be all that tricky for apple to manage , i mean usb cables are ready available just would need a plug on the device , so you could enjoy both worlds wireless if you sit on the other end of your room , or to show of your mates a cable free desk , but if you need to work plug in the cable and keep on working undisturbed by a low battery , and charging would work too that way, but again its just me who thinks economical , because batteries even rechargeable ones cost money, plus for the later you need a charger too and you need a set of charged ones ready if you want to continue working