If you are going to go for a blu style e cig, after using a pen style, I would recommend Volcano instead. They are basically the same (L88B) model, but Volcano has way better customer service, super fast shipping, and more flavor choices. You might be disappointed with the L88B battery life gives about 30 mins or so of continuous use before it needs charging, but it's small like a regular cig, and you get the PCC with the Volcano kit, and 2 batteries, so you can swap them out between charges, and they do charge quick. They also will be releasing a 510 model (Magma) that also comes with a PCC very soon, so if you are not in any hurry, you might want to hold out for that one, and get larger carts w/ longer battery life (1-2 hours)
Blu is ok, not bad for beginners,prices are good but very slooooow to ship. If you are about to drop $60-$70 on a kit, you might also want to look at the Janty eGo. It has a very long lasting battery (8-10 hours) and comes with 2 batteries, 510 atomizer and charger, but no PCC (because of long battery life) I love my eGo, the battery lasts all day before I have to swap it out for the other one.
901 is a longer atomizer with it's own threading, so it only fits 901 parts or you can use an adapter to connect it to other parts. I like the 901, it gives good vapor and taste, but it is kind of long.
510 is a shorter atomizer that fits the L88B (blu/volcano) parts, but they do not use the same charger because 510 is -/+ when L88b is +/- (or visa verse) but you can put 510 atomizer on L88B battery, or L88B atomizer on 510 battery. Again you can also get adapters to let you interchange the 510 with other e cig parts.
Most people like the 510 over the L88B, it's relatively small, gives great vapor, has a good sized cartridge and is closer to the actual size of a cigarette than 901 which is more like a pen style. You can get the 510 in a manual button style, which lets you control how long your atomizer is on for. The blu/volcano are only automatic and can be touchy, go off in noisy environments, or in windy conditions.
I hope this helps you a bit.