I don't know how these perform in real-life benchmarks, just a couple of thuoghts:
1. If you'll go with a HT processor, be sure to run a 2.6 kernel, in 2.4 there is weak HT support.
2. I would prefer a HT processor on a server with a lot of concurrently running processes (e.g. a web server, MySQL server...). If you want it for a server with only one (or relatively few) CPU intensive tasks running (such as PostgreSQL database, sendmail server, ...) or a desktop machine, I would prefer a non-HT one.