My advice is to save your money BECAUSE this is going to be one expensive habit.
The decision really should be based on what you are going to do long term. If the plan is to keep this one and buy another later the best price/performance in your current range would be the 55-250. If your going to flip the lens once you get the additional $ then the 70 - 200 f4 would hold it's value better. Either way on the 7d you should get some fantastic results.
I agree with Brad on this one. The 55-250 is quite a surprising lens for the money. I doubt I would ever sell my one, it is tack sharp and a good example. It may need to be babied a bit as mentioned earlier, but really, who does not baby their lenses? Everybody I know does.
By spending a little to start with it will also give you a good opportunity to revue your shots and work out what focal range you use most. Then you can go out and purchase the best lens you can afford within your most used range.