I'm using it in a Home Theater PC box where heat generation is more important than benchmarks (within reason)
buy 2400, turn off turbo, save money and pain.
Undervolt if you need to. 2400 by itself uses 55W at full power in this review:
http://www.xbitlabs.com/articles/cpu/display/core-i5-2500-2400-2300_10.html
705e also is using right around the same total power, maybe 5W less:
http://www.silentpcreview.com/article1056-page2.html
Keep in mind the 2400 also includes the GPU, while the 705e does not. Most likely at default its' using less juice than the 705e + onboard video, at idle and low usage it
definitely is since the SB CPUs have much better power gating to minimize low and medium power.
A 2500s is maybe 10W less at max load. These are all pretty marginal differences, and I doubt your primary issue will be resolved with these.
Most people don't actually run max load much. You can see the SCPR review shows the 703e using less than 20w during x264 playback. If you have an overheating issue, it's likely not going to be solved by saving 5W on the CPU. Most likely it's inadequate heatsink / airflow issues.
Most I've talked to who try to do HTPC quiet, but fail, either try to do too much fanless. Or buy stuff not designed to be quiet, then try to make it quiet (which usually makes it perform the original task poorly).
Fanless is really difficult & expensive to do correctly. It's much easier to have a fan on the CPU, but make sure you have a quality heatsink and the fan is QUIET. The fans designed to be super quiet, like the Nexus silent fans and the Scythe Kama low speed fans are basically inaudible, but still have decent pressure differential from front to back.
Get your airflow / heatsinking right and your CPU choice 2400 / 2500 S / T versions don't matter.
If you're overheating a 50W CPU now, a 40W CPU may be low enough that you fix your overheating problem, but it may not be too. I would look real hard at other places in your case to add some airflow quietly.
Again, if you've never used a fan designed from the beginning to be quiet, you should at least try them. They are on the expensive side, usually close to $15 each, but they do work well.