Trying to "future proof" your system can be fruitless (or at least not very satisfying). AMD platforms usually are more "upgrade-able" in terms of being able to add newer, more capable CPUs as they are available than Intel offerings. Both AMD and Intel will be moving to newer CPU socket offerings, thus if you wait for the next great thing, you never buy anything.....
Hyper-threading will have minimal impact to FSX (and gaming in general) as most games really can't take advantage of the multi-core CPUs any way. There are studies that show that many games have a 3-core "sweet spot" when it comes to taking advantage to multi-core systems.
The i5 does indeed out-perform the AMD offerings in many benchmarks, but in the bang for your buck comparison, AMD nearly always wins.
All 3 of your options will give you a good system for 2-3 years. Look at your budget and buy the "most" computer you can justify within that budget. Have fun!