Both has 7 Wimbledon titles but Renshaw has 6 consecutive titles from 1881-1886 and a further 6 Wimbledon titles in doubles. Both were runner-up once. Fed in 2008 and Renshaw in 1890.
Federer is obviously the more versatile player on other surfaces as Renshaw never won the US Open (The only other Grand Slam) but this thread is purely about grass.
Renshaw was Serve & Volley (Renshaw rush) and both were known for their powerful forehands
They also had similar rivals. Renshaw's opponents were Herbert Lawford 3x times, John Hartey 1x and Ernest Renshaw 3x. Roger's opponents were Roddick 3x, Nadal 2x, Murray 1x and Philippoussis 1x.
Renshaw's brother his rival had won Wimbledon once unlike Roddick (Though both I guess were good grass court rivals) while Nadal won Wimbledon once. (Though Nadal is not a grass-court specialist like Ernest Renshaw)
Honestly I think unless Fed takes another Wimbledon to break the tie, the title slightly goes to Renshaw
However I suppose we do have to consider that Renshaw had automatic entry to following year's Wimbledon final since back then Champions only had to wait for someone to earn the privilege of playing him in the final by beating everyone else while Renshaw sat back playing no matches. This could be a disadvantage for Renshaw since he would not be able to play himself into form in the earlier rounds like Federer often does when he starts off slow in slams.
So who is the true king of Grass?