I use BC because I can easily make a lag-reducing detector that detects if a certain condition is true and if it is it turns off the pipes for lag control.
You could probably do the same with transposers and some redstone, but I can smack a 2x3x2 detector on a timer/pipe and the updates every second are reduced.
BC and RP2 are essentially the same thing. BCs diffrent pipes = RP2s diffrent machines. RP2 pipes are slightly more smart than BC pipes, but BC pipes are much more fun to use (for me anyways) as I get to set up a complex system to make sure it doesn't muck up. An example being a bee system I have set up for an alveary that tries to put things in a chest, and if it can't, then it doesn't jam the system. It just flies out into my hallway of transposers and gets sucked right back into the system. Yeah, it's probably laggier than a RP2 system, but it looks cool to see bees falling down my waterfall, and then going to a chest and seeing everything get sorted.
Also, I don't like to use the same mod for everything. Item storage? Barrels. Tools? Omniwrench. Flying pigs? Trail mix. You can see that all my needs are supplied by diffrent mods, and not by just one mod called factoolization mix that does what all the other mods do already.
EDIT: Well then, that's quite funky. As soon as I post this, I resume my Direwolf20 video and INSTANTLY the first thing he builds is a detector to reduce lag. He's judging my ideas Q.Q