BF3 and Skyrim both use software audio. (Pre-BC2 Battlefield games and Morrowind would sound better on the X-Fi, though, due to their use of hardware sound acceleration.) Racing and flight sims tend not to reveal much of the advantages of positional audio compared to first-person titles, though it's still cool to notice when someone just flew over or under you using nothing more than sound.
Looking over your first post, you said you already set a maximum bid on the Essence ST, which likely means you've already committed to buying it legally, unless someone outbids you. I suppose you can just wait and see if you win the Essence ST or not, and if you don't, go buy the X-Fi Elite Pro if it's still available.
(Also, unrelated note: are any of those flight sims IL-2 Sturmovik: 1946 or Rise of Flight?)