You mentioned Weight training logging and calendar management specifitally, so I'll base my answer on that:
On both devices, the out-of-the-box calendar functions are pretty basic, but somewhat similar. Palm is faster, PocketPC has (I believe) a bell and whistle more, perhaps. If you wanted some more sophistication, you would need a third-party program such as the long-established Pimlico DateBk6
for palm or Pocket Informant
for Pocket PC. I have found PI to be much more flexible and interesting than DateBk6, but, as I keep mentioning, slower.
I don't know much about weight training logging software on either platform. When I was researching it for myself on the Palm in 2003-4 I found available software to be inflexible, clunky and over-priced. For all I know there could be a killer app there now, for either or both platforms. And both platforms will sync with spreadsheets, so you could go that route. Otherwise, the basis Palm memo function is very fast and easy but has no formatting at all. Pocket PC handhelds can read text files more easily.
The chances are that which platform suits you best will be more to do with your current set-up and costs than which is overall "better". Also, what third-party software appeals. Also, if you are considering taking the machine into the gym, how sturdy it is.
As to taking into account future phone upgrades, you probably don't need to worry too much about that. I transferred from Palm to Pocket PC (with some annoying hours of work) lately, and although I coughed up for new Pocket PC software, I only spent $20 (I think) on software to help the conversion along. As long as Palm has a community, you'll be able to get your basic stuff off it. Just watch out for third-party apps on either platform, in case they lock you in.
If I were you, this is what I would do:
Decide on a budget
Check out what is available to buy in palm and pocket pc within your budget
Check out software on both platforms for specific things you want to use it for
Work out if there are bottom-line things that are very very important to you, and research them and/or post here about them. The platforms vary in so many small ways, but if a couple of those ways coincide with your must-do's or must-not's, if could helpfully inform your choice. Such things as syncing with PC data, reading data from and file storage on memory cards, publishing or sharing of calendars, and compatibility with different desktop operating systems, are quite different.
Hope this helps.