1: CMSS-3D Headphone wipes the floor with Dolby Headphone in games that use DirectSound3D or OpenAL, because it's not reducing the 3D sound space to virtual 5.1/7.1. Instead, it's closer to an actual binaural presentation, complete with height cues.
But in games with software audio engines that downmix the sound to 5.1/7.1 to begin with, they're about even. If those are the only games you play, then the X-Fi cards lose a lot of their gaming advantages since they're not using EAX 3/5 or exposing the actual 3D sound buffer.
2: Coming from an X-Fi Platinum, you'd probably get cleaner analog output, though it would be more practical just to add your own external headphone amp and get the adapter needed for S/PDIF to feed a DAC down the road. The only exception is if you're concerned about the longevity of the old PCI interface, in which case buying an X-Fi Forte or Titanium HD should keep you set for a while.
Still, while having a dedicated headphone amp built-in adds to the value proposition, think about this: do you match your headphones to your amp, or do you match your amp to your headphones?
3: From what I've heard, they both have their moments of suck, hence why both have modded driver packages (like this one for Xonar cards). In my experience, though, the X-Fi Prelude, Forte, and Titanium HD drivers work fine most of the time, except for some rare instances where I can't switch to the needed mode (either Game Mode for EAX 5 and CMSS-3D Headphone, or Audio Creation Mode for bit-matched playback and ASIO; I never use Entertainment Mode). In general, however, people report better driver experiences on the X-Fi side of things when they're using the newer PCI-Express-based cards, especially the Titanium HD.
4: There's Creative's new Recon3D cards, which are pitched to have dedicated headphone amp circuitry, but I'm skeptical if they're worth buying at current prices over an X-Fi Titanium HD without having one to review.
5: 5.1/7.1 speakers down the road, huh? That actually puts the X-Fi Forte above the X-Fi Titanium HD for convenience's sake. You can choose whether it mutes the speakers when headphones are plugged in or not via a driver checkbox.
The big thing that complicates things as far as recommending either sound card to you, though, is that you already happen to have one. Why do you want to replace it? Is a headphone amp the only reason?
