Well....
Guitar Pro is set up alot more easier to use... Well, they are both essentially the same but appearence wise things seem a bit better laid out and you dont have to worry about all the wanky spacing that I've run into with power tab (which is not a hard thing to fix anyway).
Guitar Pro also isn't meant just for a guitar sound, you get a whole bucketload of sounds from synths to pianos to yea... pretty much everything, and you can make drum tracks.
Power Tab def gives your tabs/music a more professional look/feel when you present it though.
And finally, the big difference between them is... Guitar Pro is a 50 dollar program and Power Tab is free. lol
I think in the newest version of Guitar Pro you can export midis as a wav file which can be useful if you want to import any of that audio into alot of recording programs.
Basically, you would only take guitar pro if you got it for free some how... like... umm... not me.... Or if you wanted to create tons and tons of crazy midis. Power Tab is def the better tool to use as far as teaching goes.