It depends a lot on your uses, but keep in mind that once you have one you'll find uses for everything you didn't think of. People put a LOT of emphasis on the weight of the wider grapples (apples to apples construction) and really it is very minimal. I think my 66" WR Long grapple weighs about 100lbs more than the 48". I can tell you that I've never been within a 100lb deficit on my lift capacity. If the rock or log was too big to lift with my grapple, it was too big to lift with any 4' grapple as well.
Another argument becomes digging. Again, I've never found that I couldn't dig something up because of an extra foot and half or so. Pretty much all the arguments you see, one way or another, are really just people over thinking it and not having experience with both. There are some huge proponents of one specific size, and I caution you that if something is diehard one way or another, take their advice with a grain of salt because they are likely overly biased.
I bought my WR Long OBG2-66 about 10 years ago. It's been the best investment I've ever made, literally the best purchase I can think of. I almost never buy new, but I had no choice and bought new with this grapple. My father has used it many times as well. He ended up buying a smaller tractor and I recommended a 48" grapple for him because his lift capacity is in the 1,000lb range.
He's had that grapple now for about 2 years. Both him and I much prefer the wider grapple for various reasons. He still jokes around about me 'screwing' him over by getting him the 4' grapple lol.
Scroll through some of the pics in this thread:
Grappling fun - A Picture Thread....
I think you'll find various grapples of all sizes on there, and likely every single thing that someone did, could have been done with either or....
My main recommendation would be don't buy something meant for a skidsteer. You definitely do NOT want one of those 800lb+ grapples. There is simply zero advantage there. If your talking about 3 point implements then yes, the heavier the better, but for the FEL, something that heavy just doesn't have an advantage.