I haven't seen too much hype for either game - they're both launching on the same date (August 20 I believe).
I might get XCOM for the 360 around the launch date; it looks pretty solid based on the gameplay trailers I've seen, although I might wait until the Steam winter sale or such based on reviews before getting Splinter Cell.
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I'm getting Blacklist. Spies vs Mercs is back and one of the best MP experiences will hopefully improve. I'm on the fence with XCOM and will see how it is received.