I'm very sensitive to weed-- if I have more than about one hit, I get full-blown panic attacks, depersonalization, nausea, etc. It feels like I'm in a dark, terrible place I can't get out of. No one I know has such an extreme reaction. MDMA is completely different for me. I get a little anxiety on the come-up, but it's more like the "jitters" rather than fear. It feels like I'm hyper-aware of everything around me, shaking, and a little nervous-- but the big difference is that it's not uncontrollable (I can distract myself easily, and I still feel like I'm in my right mind), and it passes quickly. Then once I'm up, it's practically impossible to feel anxious, even if I try.
Different chemicals affect everyone differently, so it's hard to say how you'll react. That said, MDMA (if you have the real stuff) is known for having fairly consistent effects on most people. It pretty much pushes you into a positive headspace, as long as you aren't resisting it. So, if you're relaxed, thinking good thoughts, and in a comfortable environment before taking it, there's probably little chance that it will trigger your paranoia. Just start with a reasonable dose (100mg is good), grab a friend or two (or even better, a significant other), put on some good music, and try not to psych yourself out about it before it kicks in.
(wasn't always the case) do you think an ecstasy would be any different? 
