I've shot some of both, and the first time I used Provia 400 I too was very pleasantly surprised by how sharp and grain free it is, but I later also discovered that it isn't until you start making larger prints from the material that the 100 starts to really show its advantage in resolution and grain. On the other hand, the 400 makes more types of photography possible with slide film, like hand holding the camera becomes infinitely easier and accommodating. I've never push processed Provia 400. With such narrow latitude, it would seem that you would bury your shadow details pretty quickly, but if you're in really flat lighting you might get away with it.
With 645 negs - what do you do with the resulting pictures? Do you scan and print, or do you mostly use digital files as your final destination?
The reason I'm asking is that you must consider what your final use of the images is, and then take both films to their limits to see what's possible, and then decide for yourself. To make a fair comparison you should probably use a tripod for the test, and then count film speed, graininess, sharpness, and so on in a pros/cons list to see what actually matters to you.
Thanks for the advice. So far I just store these images on my computer. I dont know if my scans are good enough to print but I'll try one just for fun. They look decent on the screen.