One vote for the iPhone here. AT&T has drastically better customer service than Sprint, and Sprint isn't doing so well as a carrier, their CEO just resigned. That translates as "things aren't going so well around here." That generally means that things aren't likely to get better for you as a potential sprint customer.
The iPhone does a lot of things that the 755 won't do. Web browsing is the biggest moneymaker for me, and it opens up a wide swath of possibility that just isn't there on the WM or PalmOS platforms. Music playing, video watching, iTune song buying, podcasts, etc -- the iPhone is better hands down. If you want business class email, the 755 will probably be better. If you're looking for personal email (gmail, yahoo, AOL, etc), the iPhone will be easier to setup and use, though not as powerful as the 755. Actually, that last phrase translates well to the entire experience of both phones.
The iPhone is likely to see software updates come fast and furious and the software experience will get better over time; the 755 is not. This is offset in many regards by the ability to install many 3rd party applications on the 755, an ability that the iPhone currently lacks.
There are programs you can put on the 755 that you can't do on the iPhone (yet, or unless you're willing to hack the iPhone a bit), such as instant messaging, and the aforementioned office editing. The iPhone cannot do GPS at the moment, though hacked iPhones are able to do cell phone tower triangulation with the purchase of additional software.
The form factor of the iPhone is better, as is the battery life, as well as the screen. You don't have to worry about obsolescence of SD card formats either, which becomes a hassle if you're a phone collector. No one is likely to call you an 'Apple fanboi' or the like if you go with the 755.
Full Disclosure: I run Phone Different, TreoCentral's sibling iPhone site and am obviously biased.