Interesting question. I'll give some reasons as to why I choose one over the other.
Brock Lesnar is a better athlete than Cena. Only because Lesnar trained differently than Cena.
Cena is not so shabby himself. People are tired of Cena unfortunately. So he gets a bad rap. People consider him sub par. But they need to realize that this is staged wrestling. Every wrestler has their 5 moves of doom etc. Just like Hulk Hogan, people sometimes hate on him too for not being a great wrestler, which is mostly true of him however. They need to realize that WWE limits certain wrestlers and their moves and skill set.
If anyone has seen Cena when he was the Prototype (He had way more moves, was very aggressive and had the mouth to back himself up). When he first started in his Debut match, he wrestled Kurt Angle for the first time and realistically was put through the test. If you get a chance to watch this match, it is pretty amazing.
And realize Cena was just a rookie at the time and was his first WWE Match. Usually Angle would blow through these matches with the Angle challenges. Cena went all the way through the match and almost won legitimately.
Is Cena perfect? No. But he still proved himself.
All other wrestlers have their 5 moves of doom as well. HHH, The Rock, Stone Cold, Undertaker etc.
These moves need to be limited, otherwise people will get legitimately hurt. Wrestlers have to choreograph some of their moves with the person they are going to wrestle.
Cena has great work ethic. He is and was around for all house shows especially during his prime.
He works well with most of his peers and is respected. Lesnar is and has been known to be a flake.
Lesnar is not really loyal to the company. How many times has he left and then come back for only because of the money.
Cena also does a lot for charity and works hard to help make things happen for kids who are in need. Through Make a wish foundation. He also supports the troops.
Again Cena may not be perfect but he has always stepped up his game to the best of his ability. And has great work ethic.
So my vote is Cena.