There really isn't that much of a difference except for the fact that Eli has put up "good enough" numbers each year he's played. I also think that Eli has more potential than Grossman - there's no way of actually putting this into words, but Eli makes some throws that only one or two other QB's in the NFL could get away with..... of course, the next time he drops back to pass, he'll throw it right into the chest of a defender.
Grossman had some injury problems starting out his career, and I really liked him his first few years - I thought he was going to be a great, great QB. However, after watching him in 2006, I changed my mind. Look at the teams he was playing against when he had most of his big games that season - is it an accomplishment to throw 5 TD's on the season against the Lions? How about 3 against the 49ers? And 2 against the Rams?
He essentially had what I'm forever going to refer to as a Derek Anderson season - he took over at the right time, and he put up big numbers against bad teams. He may have had a good game or two against mediocre to good teams, but for the most part, he'd play awful against good defenses.
Calling Eli Manning a great QB is beyond a stretch - quite frankly, I think he pretty much stinks. Don't get me wrong - as I said - I think he has a lot of potential, but so did the likes of Jeff George, Kerry Collins, David Carr, and even Drew Bledsoe. If Eli could eliminate some of his mental mistakes and improve his accuracy (in his defense, his WR's drop a lot of balls), he would be one of the best in the league - that is the difference. Eli has his fair share of bad games, and mistakes, but I wouldn't consider Grossman to be "on the bubble" to becoming one of the best in the league. On top of that, Eli plays significantly tougher schedules (most seasons). The East has been the most competitive division in the NFC for the past two years, and if you don't think the Bears had an easy schedule in 2006, you're kidding yourself.