I'd argue both were best for the teams they played for and their times.
Peak Marcelo delivered more in attacking phases. Yes Bobby had quite a shot and all that, but had a "wide midfielder" level of contribution in attack while Marcelo could be an out and out wing forward and easily one of the most dangerous players on the pitch.
Problem is I can recall Marcelo defending like an absolute clown but not Roberto Carlos. Caught out? Maybe, but not bewildered clown look.
If I had to pick one not knowing the rest of the squad I'd probably pick Carlos, but I can see situations where I would pick Marcelo. You could say roughly the same about Cafú and Dani Alves.
Debatable, I'm just the sort that thinks you can't lose if you don't concede and builds from there. Does Brazil win the 2002 World Cup with Marcelo and Dani Alves? No chance. There's no way Roque Junior survives having Marcelo at LB, unless you ask him not to play like Marcelo, which would be daft.
Edit: when I say "wide midfielder" level of contribution it's still substantially more than any wide player we've had since Nani, mind.