I believe the Bushnell Holosight II and Eotech 500 series are idential except for the branding. The 510 series adds a metal protective hood. The 550 series adds additional night vision brightness settings, and in addition to the two up and down buttons controlling brightness, it has a button that toggles between the night vision and normal brightness levels with a single press. The 552 uses AA batteries, all others use N batteries. The AA version is bigger, but you get more running time out of it, I believe 100+ hours vs. only 75 with the N batteries.
I have the 510 series myself - wanted the extra protection of the hood, but don't have any NV equipment. I figure by the time I can afford anything worth using with these scopes, they will be dirt cheap anyways , or the next greatest thing will be out.