personal preference really.... but why not have both xD 1080p, ultra preset, sweet fx, pretty much always 100+FPS for my 120Hz monitoronly time it really dips is if I'm standing right on the leveloution..... then it hit's like a *****!!!! but that's more the games fault and not setting changes will help that ( both myself on max and my mate on low had god awful FPS drop when standing in close prox to **** collapsing, damn, hotel, etc... but a bit further away and no probs really)
I use a mixture of settings not only so I can hold 125fps capped solid (except in leveloution) but also to get the clearest view - full ultra for instance darkens building interiors more (partly due to ambient occlusion) than other settings when looking into them from outside.