The difference between Timtex and Decor Bond is HUGE, in my opinion. When I make my bags, I use the Decor Bond right next to the fabric, just to give it some body to keep its shape. You can use the Timtex or fast2fuse on the inside, fuse it to the Decor bond to give your bag more substance. I've tried making the bag without the decor bond and somehow, it doesn't hold up as well. You could try making a small bag sample to see what I mean.
Hmmm...do you mean to fuse the Docor Bond to the Timtex and then use it as a sew-in interfacing? If so, I think that I like that idea because no matter how careful I am when I fuse, the purse will usually look wrinkly after it's made.