I think that Zhang Fei's fight into Shu following Liu Bei is very impressive. He took initiative and showed some real tactical thinking. He was strong and capable as a general.
Guan Yu was charged with more responsibility and ultimately failed in those.
Who's to say if they changed positions if Guan Yu would have been just as great as Zhang Fei when going into Shu? Also, what would have happened if Zhang Fei stayed in Jing Zhou? Would the outcome be better or worse?
My feelings is that they're pretty similar in quality, I feel Zhang Fei did better when on his own in the battlefield than Guan Yu, but Guan did some pretty amazing stuff when he dueled.
Guan Yu also seemed amazing when he had a leader to guide him- being on his own with just a bunch of nobs in Jing Zhou was pretty disastrous for him because I feel he needed quality guidance, and guidance he would respect.
So I think they're equal all in all.. as far as I know!