Honestly I couldn't tell you the difference but I used to use AGUF awhile back as well and have since switched to pitbull as well. Personally I don't see a reason to switch for two reasons:
I have pitbull set up the way I want it and don't feel like mucking about with something I would have to set up when what I have works fine already.
And finally Pitbull does exactly what I want it to and doesn't seem to be too heavy on resources.
I would say to you stick with what you feel more comfortable with. Sadly I know this probably wasn't the answer you were looking for but figured it might be of some use as I have had people tell me that this addon is better than the one I was using and have swapped through many many of the big addons in the time I've been playing WoW and honestly when it comes down to it just use whatever you like/feel comfortable with. I doubt there is much of a difference between the two other than a few minor things that probably aren't that big of deal, like I said its been quite awhile since I have used AG Unit Frames. Anyways I tend to enter off topic zones if I don't cut myself short.
