I don't know how anyone can make a clear judgement unless they've ridden both a lot anyway.
But comparatively...
I loved Mamba's first drop. Nemesis doesn't have one. Okay, so most inverts are going to start more like Mamba, which is why I mentioned that maybe if I had more to my name things would be different? You don't ride a new coaster and slip it into your top ten comfortably, it should completely reshuffle your list because you've learnt something new. Having an unusual ride as your first of that type can't ever be a good thing. You don't really know what to expect from future rides of it's type or what it's doing well or not so well. Similarly, when I first rode Inferno, I preferred it for a short while. Will it be the same with Mamba?
The most important thing for me, though. Is that if you ride Nemesis at the start of the day, it sucks and if you ride it at the front, it's utterly force-less and dull. We rode Mamba, front row, at the start of the day... and it was amazing!
Then there is the zero-g problem. Until Mamba I could only guess what a zero-g should feel like. Inferno's is a lateral mess and Nemesis' produces negatives. This element is supposed to be what makes an invert and it sucks on the highest rated invert in the world?
I loved the little immelman. I couldn't help but think that Nemesis stall turn would be so much better if it did that. I really felt like Mamba was literally changing it's mind about which way it wanted to go last minute, which is how a tiny relentless coaster in a pit like Nemesis should feel.
I hate Corkscrews. Fly overs, whatever they call them. Mamba's were just as dull and boring as every other Corkscrew. Nemesis wins here, because it's are in interesting places. Simple as. You come off that non-existent first drop and power through that corkscrew and it's brilliantly weird. It's taken a crappy element and made it amazing.
I thought Mamba's last turns were gunna suck... but at least one of them was really... swoosh...y? It was like a bobsled. I know that sounds weird, but you felt the need to move your whole body with the directional change?
Speaking of bobsleighs, Europa's did an' half suck. Blackpool's is so superior.