Hm, the Genesis graphics look much better in those screens that awack posted. I hope the PCE one has decent music.
I think that goes to what i and black tiger were saying about some of the art work being better in the genesis version, to be fair, the genesis one wont look so hot compared to the pc engine Altered beast on a TV due to the fact, for example the screenshots i posted have color counts for the pc engine AB of 63, 60 and 46, the genesis AB are 38, 36 and 27...so the genesis will look a bit washed out.
I've owned the Genesis version since it was first released, so I'm pretty content with its visuals at this point. I'm sure the PCE one has nicer colors, but man, in those screens you posted, the PCE's stage 4 background and those rocks in stage 3 look rudimentary and absolutely awful.
For what it's worth, I don't think AB is a good game to begin with. The only reason I've held on to the Genesis cart is because I like the music and it has just a bit of nostalgia value for me. The more I read and see of the PCE version, the less interested I am in getting it. But curiosity will eventually win out.