Promotions are great and promotions help units survive and achieve much more than a green unit would, but promotions also get very expensive after the first 3.
In Civ IV, I generally try to crank out a decent number of units with 5 XP at production and then I put 4 of them under a Great General to allow all of them to hit 10 XP together. That gives me a nice balance of high XP and a good number of units getting that XP. Because CivIV BtS has a lot of outstanding promotions that occur on the third promotion (City Raider III, Woodsman III, etc), getting to 10 XP is very nice.
In Civ4Col, I'd probably either go for 2 or 4 units. Most of this depends on what I'm going to do with the units. If you got a Great General, then you were involved in a war. I'll assume it was before the Revolution and that you are going to continue with your wars, so...
4 Dragoons, each getting 5 XP. You probably have 4 Dragoons that have 5 XP already, so taking them each to 10 XP gets you the extra promotion quickly and makes them just that much more effective in combat. If you don't already have a Scout up to 5 XP yet, maybe make one scout with Explore II to get the faster move through terrain and the greater visibility. He won't win any serious combats, but he will give you a much better idea of what to expect near your combat and when you're finished this war, he'll give you an excellent idea of where your next war is going to be.
I'd most likely make mostly the same decision regardless of my leader choice - even half cost XP means 5 XP = 3 promotions, so I might drop the promotion on 4 brand new Dragoons to get them up to fighting fitness quickly. Otherwise, I'll go for 4 units, each of which already have a few XP so that I can get the most bang for my buck.