Just as a note, there's also the UFO series that is <ahem> "inspired" by XCOM. Not sure if it's a spinoff, or some company forming from the guys who made the original and wanted to remake it, or a complete ripoff, but the feel is very similar - a combination of base management & research on a global level, and tactical combat on small maps (pausable real-time, like what was introduced in X-Com Apocalypse, rather than turn-based).
The three games are UFO: Aftermath (a bit short and samey), UFO: Aftershock (good range of enemies, been playing it for a while now and it doesn't feel stale) and UFO: Afterlight (not got round to trying this one yet). I picked up all three plus Stalker for 15 quid. Well worth while, if you liked the original UFO: Enemy Unknown.
Oh, and if you want to stick with the old-school games but find UFO:EU a bit easy, try XCOM: Terror From The Deep. The "tough soldier" alien race is almost unstoppable, the powerful alien guns don't have handy auto-fire modes and the the Heavy version only has a 10-shot clip, you need all sorts of crazy stuff to get anywhere in the tech tree, the maps are huge and genuinely spread across up to four levels, there are several missions spread across up to three separate maps where you only carry through whatever soldiers and equipment managed to get to the exit of the last map, and to top it all off, a fair number of the missions take place out of the water, which means that half your weapons are useless and nobody thought that it might be an idea to give you any normal guns for this eventuality.