Hi and welcome to Everything Treo!
You really have a choice between either the Treo 680 or the Treo 750v. The obvious difference being the OS and the Treo 750v's support for 3G. Both have a similar form factor, but I can't speak to the build quality of the more expensive Treo 750 as I haven't seen one yet in person. Palm uses different manufacturers for each.
From a Mac perspective, the Treo 680 is Mac friendly "out of the box". Upgrading to a new Treo 680 will be seamless and you'll also be able to use all your 3rd party Palm OS applications. The Treo 680 also has a new Phone app that integrates favorites, call log, dialpad, wallpaper into a tabbed interface. Very nice! On the flip side, you have the Windows Mobile OS. Generally speaking, the OS requires more navigation to get to things versus the Palm OS. That being said, it is very intuitive and feels like a portable version of Windows. In order to sync with your Mac, you'd need Missing Sync for Windows Mobile. Mark/Space is the only current solution to sync with your Mac. I haven't used Entourage, so cannot speak to the sync capabilities using Missing Sync but it is supported. I have had no problems to speak of using Missing Sync for Windows Mobile with both the Treo 700w and Motorola Q. The only downside to using a 3rd party app is that you might incur bumps in the road.
When
reviewing the Treo 700w, I did a number of comparisons between the Windows Mobile based Treo vs the Treo 650. You might find this useful when making a decision on the OS. Here's a
review of the Treo 680.
Good luck with your decison.
-Chris