Start saving your pennies for either the efs 17-55 2.8 IS or the efs 15-85. Buy lenses (preferably used) for your current camera and don't concern yourself with a future "what if".
I've struggled for some time to decide which lens to buy. I bought a 60D with the 18-135 lens which, despite its versatility, is neither sharp nor fast. About a year ago I bought a 50/1.4 for portraits and low-light, but I've found it to be not wide enough for casual shooting. About two weeks ago, after considering going FF, I've decided to follow such advice and get the 17-55/2.8. It's a wonderful lens, that stays on my camera 95% of the time now; only very rarely I need shallower DoF than it can give. (When I first started it was always about razor-thin DoF, but nowadays I prefer both eyes in focus...)
I guess the most important thing is to decide what your needs are. If you need the best low-light capabilities ever, then you should go FF. In my case, I wanted to go FF (it happens eventually after spending too much time here...) but had no need for it, and couldn't justify the expense; I sticked to what I had, for it's more than enough for a little landscape shooting now and then, and for taking pics of family. By the end of the year my kid will be walking around the house, and only then I'll buy another camera for having better AF (70D or 7D mark 2, will see).
The grass will always be greener... Be happy with what you need/can afford.
Make a decision based on your needs and buy good glass, and take good pics now. Don't spend two years using inferior glass because you might go FF some day, like I did.