I think the 2 are completely different workouts. I own both Tonique (all 3 of her DVD's actually) and Butts & Guts. With Tonique, it is VERY high rep (I think she does a minimum of 30 reps of everything) and in some of her DVD's you also use handweights, but she says to only use 3 pounds. She factors in some upper body movements with these weights, to get more of an overall workout. With Butts & Guts, it's lower rep than Tonique, and heavier weight and truly focused on Butt/Legs and Abs.
I like both DVD's however I grab for Butts & Guts more than Tonique. Tonique is a long, constant burn for me, and it makes me dread it a little. Sylwia is a good instructor, but not the best at cuing and can be hard to understand due to her strong accent.
If you haven't already, you can preview Tonique workouts on her website, I believe.
Hope that helps!
Erika