Both of the two systems you mentioned are very high quality and come from well-respected manufacturers. To me, the Akra comes off as being a little more exotic, but that's just my opinion. Honestly, even if there's a slight difference in power between the two, it'll be so small you probably couldn't tell it. Both systems will have a modest sound at idle, and a nice scream up top. Going on what Do3 said, Yoshimura used to post their dyno charts on the website, but I'm not seeing them anymore. Akrapovic still has their numbers posted, and if memory serves, they are just a little better than the R77.
Get a good tune (don't listen to anyone who says you won't need it), and you won't go wrong with either system.
Also, here's one that I personally like a lot, but very rarely see on the road...
M4 Exhaust - Gen II Hayabusa
No matter what you decide to go with, call Justin in the parts dept at Honda East Toledo (419) 891-1230. I've done a lot of business with him over the last year, and he's always great to deal with. Super fast shipping, the best prices, and he's a Hayabusa.org sponsor.
Oh, and remember to post pics no matter what you get!
