OK I have had a look at both and am now able to give an opinion about both.
Guitar Pro you have to pay for, but there is reason for that. It is an easier program to use and it has more features that make using the program user friendly. Guitar pro alows you to put down numerous tracks each with its own instruments, there are quite alot of instruments available. Drum tracks, string tracks, precussions, sax, trumpet etc its all there. What is nice is the fact that you can set the volume individually for each track and therefore mix the song to your requirements.
Power tab is a freeware program and was written acouple of years ago. there are a lot of free
PT tabs on the net but they are not available at the moment, because of some legal squirmish. PT only alows you two instruments on a tab, guitar and bass. Changing the time i.e. speeding up or slowing down the song is difficult ( i havent figured it out and have given up trying.)
My final vote would propably be for GP seeing that the program is easy to use and alows more tracks. The fact that you are paying for the program is not very nice. Especially since i found out that if you own version 3 and would like to play version 5 tabs you need to buy version 5 again... Something like microsoft is doing with their software, but with GP you are not able to even open the next versions tabs with just some features disabled.