I've not used the Diamond Touch, but many devices very close to it, including the original touch. and some with more power and bigger screens (a Toshiba with a 800x480 screen.
I got an iPhone a couple of weeks back and havent regretted it for a second. In fairness to the Diamond touch, which is just a high end WM6.1 device with a nice interface tacked on top, it is a great platform for many things and i do like WM6.1, but it simply isnt great as a phone interface without stylus. The keyboards often cover up vital things on the screen that makes it an ergonomic nightmare. As a pda, with a stylus, and a hardware screen its great, but as a phone, with soft keyboard (even though the new htc ones are close, and apples is not perfect) the apple one feels so much more designed for it from the outset.
Opera 9.5 is good however, but slow! for most browsing on wm6 your better off with Opera Mini 4.1 - which is scary fast and great on a vga screen.
The problem is, lots of web apps and pages are optimised and work great on the iphone, and whilst there are many many more apps of a wider range for WM6.1, the iphone will catch up, and they seem to work better on the iphone, and are far easier to install, not to mention easier to buy, apple has really nailed the delivery system and purchasing for iphone apps, they hit a home run with that one to use an americanism. That far outweighs the closed shop issue of app store for me.
In WM6's defence its probably still far better in the workplace / power users pocket. It has office apps built in - i do miss spreadsheets and prey for the day google docs works on the iphone (sorry know i dont know if they are actually working on it)
My final thought is the touch is often free on contracts that mean full price for an iphone, and they have no other choice because the huge hype (reasonably well deserved) of the iPhone.
My summary, iphone is amazing and wins in most areas, but if you like to fiddle, are a windows guy (or gal) and can live with a stylus, then the diamond is a great phone - but if i had to have one i would be waiting for the diamond pro with slide out keyboard
Stephen