The Titan II is a good phone but doesnt stand out for me, its just a big phone. The Lumia is stylish, and that screen is truly beautiful with the blacks widely acknowledged to be better than the Titan. Cant argue with that outdoor visibility either. I'm on my second one, returned the first because of poor battery and the replacement is better in that regard but has the purple tint, but that has been marked for remediation so I'm good with waiting for the fix. No other problems with my current device, got it in a gel case, no screen protector (messed up 4, cant be bothered anymore) and it looks and feels fantastic. Camera is obviously worse but usable enough, and judging by the 800 its not too much to expect some improvements with updates so that's not a big issue right now.
Personally, its no contest, I would choose the Lumia over the Titan II anyday as there's nothing it offers that I feel is worth switching for, definitely not a CPU increase of .1Ghz. Just too much goodness that comes with the Lumia 900 to give it up.