I like both of them as alternatives to my favorite set (which is in deathwatch mode now anyway…).
As a general comparison, Crusader uses more traditional mechanisms, while Impetus is more innovative. Both will meet your prerequisites for time, space, and complexity.
As you already have Crusader, I would recommend getting Impetus. There is no reason you cannot play both: the basing can be quite compatible, kluging the smaller Crusader bases into the large stands for Impetus. As long as both pairs of armies are based the same (especially as you're doing 10mm, so won't run across too many others), adjusting the base sizes won't matter.
If you're not ready for that step, download Basic Impetus and the pertinent lists for free and see how you like the mechanisms. They're only slightly more complex for the full ruleset, IMO, so will be an incrememntal learning experience.
The two rulesets give a different feel, and that's not a bad thing. Sometimes you may feel more like one, sometimes more like the other, depending on the period. Given your army choices, I would tend to prefer Impetus for NKE and Hittites, Crusader for early medieval, and either for Romans and barbarians; but I would happily use either set for any of them.
Hope that helps.
Allen
paints brigs for the British fleet.
