The limitations in repeat alarms is endemic to the operating system, but there are programs to fix that. One free program that can do repeat alarms is the free apReminderPlus. It can also ramp up the alarm volume. spbPocketPlus can do the same thing, but it does much more and isn't free. I'm using the repeat alarms in PocketPlus right now.
Advance alarms are included in Pocket Informant (days, hours, minutes). I know that the newest version of PI added some stuff, but I haven't had time to plumb the depths.
Either way, the setup is quite different from datebk, and I believe that's due to the differences in platforms. Microsoft is also much more helpful to developers than Palm ever thought of being, and that allows some great functionality without huge effort reverse engineering the Palm OS APIs, especially the Dynamic Input Areas. As you'd expect, PPC programs tend to work much more like your desktop apps than Palm apps do. I guess you've read my series here on MTR about my journey from Palm to PPC. That covered a lot of the differences.
You can download and look for your desired features in the PI documentation. I'm happy to answer any ambiguities that you may find there. My brain fills up faster these days than it used to, and I've forgotten more about most programs that I've used than I remember if I don't use features regularly.
The hardware of the X50v and X51v are almost identical, with the major exception of the amount of internal flash memory. WM5 works differently than WM2003SE at the system level. In particular, the install system stinks in WM5. I beta test for quite a few developers in WM, and WM5 generates the most installation problems by far. Also, WM5 uses ActiveSync 4.x, which by design doesn't support syncing over WiFi. I'm still using AS 3.8, the last version that supported WiFi because I synced that way from all over the house when needed. Although AS 4.5 adds some nice capabilities, I refuse to give up WiFi syncing while I can still keep it. Of course, Linux doesn't support syncing WM devices yet, so this point has become almost moot.
To be fair, WM5 and the X51v represent the future direction in WM. WM6 will be out shortly, with a few advance devices already being demonstrated. Someday 2003SE will drop off of the support stage long before WM5. For that reason alone, if my X50v irrecoverably died, I'd probably get an X51v. I still like the Dells for all the features packed into small packages.