Here's my "IMHO" about this:
To start, I actually prefer the thickness of the Touch Pro. The thickness of the Mogul was one of the things that I missed when I switched to the Touch. I have rather big hands, so the Mogul and Touch Pro feel much more natural to hold than the Touch or Diamond do. So, that answers the main question.
I also would be extremely dissatisfied knowing that I only have 4GB of internal storage with ZERO option to upgrade. That really sucks, considering 8GB micro SD cards are dirt cheap now, and 16GB aren't super expensive either. For this reason alone I wouldn't switch to the Diamond. I also told myself after a couple months with the Touch that I will never go back to a device with on-screen input ever again. After a year with my Touch, I got so damn fast at typing on it that I could type much faster than I can with any hardware keyboard (Touch Pro/Mogul), but it was the little things that really pissed me off. Slang words not in the dictionary, brand names, symbols, numbers, etc. Also, it was depressing to type out an entire sentence, then go back to read it to check for mistakes, and you find a couple words that got replaced with more "common" words. T9 is so friggin annoying, and I feel much more control with a hardware keyboard.
Also, I love the flash. Not just for the flash capability for camera use, but I frequently use it as a flashlight around the house at night when my gf is sleeping or whatever, and I need to navigate to the kitchen, etc. I used it a lot on my Mogul and missed it dearly on my Touch.
So...basically...all the "little differences" are HUGE differences to me. The size wouldn't matter to me anyway, because I hate pulling phones out of my jeans pockets anyway. I've started to use the carrying case. There was nothing more frustrating to me than trying to pull out my Touch out of my jeans pocket while driving. The damn cover was so "sticky" that it would cling to my pocket and it was a B***H to get out every time, even though it was so small, so it's not like I'm missing out on anything.
Ok...now I've said too much. But...you get the picture.
