I think the increased flare on the ESB is due to the fact that it may be slightly larger than the ROTJ. There is a scene in ROTJ where Fett is on the Skiff and you see his helmet against the blue sky, and you can see that the helmet definitely has the bell shape, although less pronounced.
The smaller flare may also be due to poor molding. If you look at the simplistic (not in a pejorative sense) paint job of the ROTJ helmet in comparison to the highly detailed paint job of the ESB helmet, you kind of get the feeling they had to rush the production of the Fett suit/helmet, since most of the ESB suit was lost/stolen.
Also, Brak's Buddy pointed out to me if you look at pictures of the MOM helmet, ROTJ, and the Art of Star Wars helmet, ESB, that the ESB visor looks to be a dark green face sheild, while the ROTJ helmet is a pure black visor. In comparing the photo's you can definitely seen a greenish tint to the ESB visor. The information isn't a 100% conclusive. But from what other Fett experts have pointed out to me and from the pictures I have seen, I believe it to be highly probable.
Another significant difference is that the ESB helmet has a little more of a goldish/pewter hue on the range-finder ear-piece and it also has the yellow/black/white decal on the earpiece. The ROTJ helmet does not have decals anywhere.
Also, the left ESB ear-piece is about the same green color of the helmet, and it has a decal. The ROTJ left ear-piece is completely metal/silver. There may also be a difference in the material used for the left-ear piece. This is just speculation, but it is believed that the ROTJ ear-piece could be resin, while the ESB ear-piece is metal.
On the back of the helmet where the keyholes are present, the ESB helmet does not have two round-like objects protruding from the circuit board behind the keyholes. The ROTJ helmet has two such objects coming out of the last two keyholes. (My memory is a little fuzzy on the keyholes right now, I may have this backward. If so, correct away!)
There is also a difference between the small arrows on the front of the helmet. I believe the right arrow on the ROTJ is colored red, while the ESB helmet has no color on the either arrow.
Of course the paint and weathering is different. The ESB helmet is an intriguing shade of a lightish green that looks blueish is some light and darker in others. This maybe be due to a greyish misting of the helmet. The ROTJ helmet is a darker green that pretty much stays a constant hue depsite lighting. The cheek arches are also much darker on the ESB. They are almost black with touches of grey and indigo, although the hue varies in certain photos. The ROTJ cheecks are lighter having an almost spruce green/aquamarine hue them. The ROTJ red around the visor also has more brown in it than the ESB visor red, which is a darker burgandy-like red.
Ummm, that's all I can think of for now. I'll see if anything else comes to mind.
I've learned A LOT from Brak and other hard-core board memebers. 
