Contrary to popular belief, the FX-6300 is not exactly a six-core processor. Yes, it has six cores, but it only has three floating point units (FPU), meaning that only three cores can be used simultaneously. When you compare that to a dual-core and hyper-threaded i3 which has much stronger single-core performance, the FX looks rather weak.
Like you said, the upgrade path is much better with the i3. The AM3+ socket of the FX has been around for a long time and AMD haven't released any new processors for nearly a year. They seem to be concentrating more on their APU series, which uses the FM2 socket.
Go for the i3; better per-core performance, lower heat and better upgradeability.