I'd suggest the Optimus 2X, for quite a few reasons.
Its cheaper, 1GB RAM is pretty overkill for a phone ( Like the SGSII, but not Atrix, cuz of the webtop thing ), not to mention it is DDR2 ( IINM ) and other than just having more free slow memory, there aren't many merits to it. Linux will cache all the processes onto the RAM anyway because free RAM is wasted RAM.
Next up is the processor itself. Because the O2X has Tegra 2, by default it gets plenty of support from devs but since the Exynos+Mali setup in the SGSII is new, they may be wary of developing for it or optimizing for it specifically. But then again, rumor is that Samsung is only selling the GTi9100 with the Exynos processor to certain parts of the world, and they may use Tegra 2 in the GT i9103 for the rest to curb demand or whatever it is.
As for the display, I'd happily take IPS over SAMOLED+ but a few days ago I read an article about the change in the Pentile matrix of SAMOLED+ over SAMOLED and the difference is phenomenal.
In terms of the SGSII being slimmer, with a phone that big and light and THAT slim, I'd be extremely careful. It may just snap in your pockets while sitting down or whatever. I had a Milestone and now a G2 and I love the feel of the phone in my pockets. A phone that weighs half mine which is that big, is a little unnerving. But thats just my personal opinion.
Now for my last point, I'd like to mention that I'd get an O2X simply because Cyanogen bought a T-Mobile G2x. So there's guaranteed Cyanogenmod support, which is quite possibly the best performance stock Android user experience that one can get. I had CM installed on my Milestone since it was ported around September last year and now, there are just those small features that I miss when I use other ROMs. Not to add the performance boost is brilliant. The general consensus is when buying into Android, you'd want to get the phone with the best dev support, or at least thats what I believe. I know the SGS had massive sales but the power users were having problems porting ROMs and all that, and the SGS is finally included amongst the supported devices. There's no telling how long it would take for Samsung to release new firmwares and kernels what with all the optimized drivers and what not.
Anyway, this was just my $0.02.
