They are really different athletes that utilized different techniques, I love both of them
Prime BJ and Petr have excellent MMA striking, Petr has more variety, and I think has shown more intelligence mixing up his shots, not only the type of strike but also the target head/body/leg, he's just an extremelly tricky striker, absolutley evil in the clinch/close range striking exchanges, tricks you thinking he's clinching but he's setting up a strike. BJ was just plain jabs and hooks, but don't forget he got the best of the standup exchanges in 15 min of battling GSP, ultimatley lost but had many good moments in the standup exchanges with Machida, and against so many good fighters gave them fits trying to land anything - Florian, Diego, etc. BJ also showed in many fights a willingness to adopt a wrestling/BJJ based gameplan like against Ludwig, Jens 2, Gomi, Fitch, and found himself on top during scrambles/awkard moments against Hughes, GSP, and Machida. Watching Prime BJ and Yan scrambling would be quite a treat, it would be like Rd 1 of BJ vs Nick Diaz, very fun stuff. I wonder how Petr would do if BJ got on top of him and started trying to trap his arms, land giant elbows, etc.
Also, Petr Yan is very mentally tough, willing to fight in big wars, good stamina. BJ is more of a frontrunner and the only time he's ever come back in the second half to win a fight he was losing was the boring fight against Renzo. BJ tends to fade and lose in the wars he's been in like Jens 1, GSP 1, Hughes 2, Diaz, Fitch. The first round against GSP 2 he was doing fine then he got completely overwhelmed.