That's a tough one. If molly really is molly, then it's an easy decision...take molly, but when molly turns out not to be molly or just molly, it's quite a disappointment. I got the best molly ever from the same source 3 times in a row, then on the 4th time it was molly, amp, and pipe...that really sucked!
You gotta test every single pill or powder and every single new batch. If you don't have a kit, at least learn the taste test for pipes if you don't already know it. There's a distinct difference in taste between MDMA and piperazines. Just put the pill or a little of the powder in your mouth and suck on it. If it tastes like battery acid and makes your tongue go kinda numb...it's pipes. I'm sure you know the MDMA bitter taste by now.