I don't have either of those lenses either...in fact I just ordered my XT this last week, but with that said, I've also been doing my homework trying to decide what to get for the new camera. From most of the reviews I've read, the performance between those two lenses is pretty similar. The "budget super-zooms" as they call them tend to be a trade off (quality vs. price) and certainly seem to have their issues regardless of which brand your talking about. I'm probably going to end up with a Sigma in that same range myself until I can afford something better.
Now please keep in mind that I'm not saying that either or any of these lenses are "bad" by any means...for the price
I understand they're actually pretty good in fact and all the sample pics I've seen over the last week seem to support that. However, do NOT expect any of them to perform like a $4000 300mm Canon prime. From a technical standpoint, both of the lenses you mention are probably going to be subject to at least a small degree of distortions (such as barrel distortion on the tele end and/or pincusioning, etc.). I haven't really seen much of a problem with "chromatic abberation" with the inexpensive lenses like I used to in the old days, but it's certainly possible too. Like most inexpensive lenses, any problems you encounter will probably be at one end of the lens or the other (extreme tele or extreme wide angle). If you check the internet as well as some of the photo mags, you should be able to find some fair and honest reviews on both lenses.
Now I should mention that a lot may also depend on how you choose to use the given lens. For example, in another recent thread here on OP, we were discussing the Sigma 70-300's and it was mentioned that this lens tends to be "a little soft around the edges". For some people, that could be a serious issue, but for me since I shoot mostly animal/critter portraits with a pretty shallow DOF anyways, softness around the edges wouldn't really bother me at all! LOL!!!
If your expectations are realistic, you should be pretty pleased with either lens really but if your looking for "champain on a beer budget", you probably won't find it here
Alrighty, good luck and I hope this helps you some! Let us know which lens you decide on and how you like it once you get it!
Bright Blessings & Gentle Breezes,