I have taken both two years ago, one immediately after the other. ITT requires three dives, and I spent about 2,5 hrs in the water, both pool and sea. GUE/F requires six dives, fair enough, but I spent 32 hours in the water, both pool and sea. I can't even say for sure how many dives we did because it was a blur, especially the pool lessons which were conducted in a 15*15*15 ft reservoir.
There is very little theory in ITT. They introduce a few concepts but reserve proper explanation for later classes. Decompression, gas management, and dive planning modules of GUEF are quite adequate.
Drills such as gas sharing, as well as various propulsion techniques were shown by the instructor in ITT but practiced dive after dive after dive during Fundies. The focus of ITT was to familiarize with gear, whereas the focus of Fundies was to create muscle memory for critical procedures, and to reliably control trim and buoyancy in various conditions, e.g. under task load.
In my case, Fundies was three times more expensive and thirty times more valuable. The class was very intense, all five days were long, pretty much 8 am to 8 pm, and very exhaustive. Think of it as scuba boot camp a.k.a. extreme makeover. ITT was more like Discover Scuba Diving 2.0. YMMV.