If it were me, Hurode, I'd save the money for epic flying first just to get over that hurdle. It'll also help you get a much faster mount for any gathering or dailies you continue to do in Outland (and of course later Northrend), and the speed will give you a lot more time to get other things done. With that said, a lot of people find it psychologically easier to take care of the littler things first, so paying for Cold Weather Flying may provide you with a little boost and incentive to keep going for a really great mount.

How easy will it be to save? Well, you should be able to afford Cold Weather Flying easily at 77 if you don't go hog-wild on expenses in Wrath, and if you still find yourself in Northrend a lot more than anywhere else you can use a flying mount, CWF is a better initial investment than epic flight. But I really can't stress this enough; keep saving to buy epic flight ASAP. Northrend is huge and you will shave so much travel time off by having an epic flyer. Send all of your BoE's and white items to a banker alt and make that toon earn his keep!