MDMA is MDMA, whether it's in the form of a powder, crystals, or cute pressed pills- if the only active ingredient is MDMA it's all chemically/scientifically identical.
With pressed pills there is the benefit of being able to identify them on EcstasyData.org and pillreports.com, because a lot of helpful people will reagent test and post so you have an idea of what you are getting into. With "molly" powders/crystals, there is a MUCH higher rate of "fakes" and contamination (anyone can throw some filler powder into a bag to make more profit). When acquiring any MDMA it is absolutely essential to test, (the Marquis is my favorite if I'm not in a position to do a full reagent, and it's super cheap:
www.dancesafe.org) but when acquiring MDMA in an untraceable form such as a powder, it's absolutely essential. If you're in an area where all the pills are adulterated, and someone claims to have "molly", that does not quite add up.
In terms of dose, every pill is different, and it's quite hard to say what dose will be right for you. Anywhere from 80-150 mgs is a good starting dose, but thats assuming what you have is all active material. Though a pressed pill may weigh 300 mgs, the actual active dose of MDMA may be 1/3 of that (100 mgs), and the same can be true with "molly" unfortunately. You can always take more, but you can't take less- I'd probably weigh out 100 mgs to be on the safe side. If you like to go hard and have tested your stuff, and you're in an environment it's OK to roll balls in, go higher but use your best judgement.