I know from an autograph collector standpoint, Favre alone on the helmet is more valuable.
I have a super bowl 31 mini helmet signed by Favre, and i took it to an autograph signing with Desmond Howard, and had him sign the other side of it. Being the super bowl MVP is what had me have Howard sign it, otherwise i wouldn't have added him onto it.
I have been collecting autographs for about 14 years, and do it because i love the challenge of trying to get sigs of my favorite Packers. I really like getting their sigs on photos i have taken of them. I have a pic i took of Donald Driver i want to get signed, hopefully i can pull it off this season.
I have never sold any of my autographs yet. I told my wife that when i die she can sell it all then!
But for sure, Favre alone on the helmet is the way to go, IF you are looking at it in a purely monetary value. Now, IF it brings you more pleasure to have more sigs on it, then go for it.
Emmitt Smith won't sign any helmet unless there are already other sigs on it. If it's a blank helmet, he won't sign it, cause he knows it's more valuable with his sig alone. So he wants to make SURE it's going to a real fan, not someone after money.
Bart Starr won't sign anything unless it's personalized to the fan, as that also makes sure it's going to a REAL fan, and not someone that is going to sell it on EBAY for big money. The personalization brings the monetary value way down. (Unless of course you are lucky enough to get a buyer with the same name as yours!)