Visually the differences between the two will be slight, to minor, to none.
Bushmaster uses 4150 milspec steel in their barrels, Rock River uses slightly cheaper 4140. You may never know the difference. Some of us are a little nutty and know the difference and wont have anything less than 4150.
Bushmaster also puts a nice dry lube inside their uppers which exceed milspec. Nice touch.
Rock River will come with a pressed on fake flash suppressor. It looks really nice IMO. Howwever, it is stuck on there with no option of getting it off short of cutting it off at the barrel which is a real hassle. Having that thing permanently on there could prevent you from installing some free float tubes. Like I said, it is a nice looking addition, but it can cut down on some options.
Also, at least as far as the 2001 LEGP Rock River M4s went, the M4 barrel was not a true M4 profile, and was slightly heavier. Has this changed at all? Anyone?