I've noticed small differences. I have V2's (the first ones to sucker me in) and V4Ls, and then cartos from various places.
Even though they have the same performance, v4l's come in all different lengths than v2's. The carto's are different sizes and different resistances. V4L has put out some new bats too... same threading the but the specs are different (longer, seem to be regulated around 4.2). I'm sure people in the 808 or the v4l forums could say the specifics about the differences. V4L offers two dif carto models "wow" is a lower resistance (more vapor but can burn easier) than the "cool carts" (better for higher powered bats like a passthrough). If you want to buy juices from the same place, v4l has tons of variety, but most of their flavors are surprisingly awful for my taste buds. V4l has lots of variety overall. Lots of color options on cartos, tons of color options and length options on bats, tons of flavors. They have some of the most attractive batteries I've seen.
But yeah, over all they're pretty much the same. I use accessories and carto's interchangeably. Customer service stuff is the main difference (warranties, good manners, etc). V2 offers lifetime warranties but their prices are solidly double that of v4l who only offer a 45 day warranty.
I'm a v4l fan because of prices and battery colors/patterns. But if I remember right, Halo's pretty good for that too.