I wrote up a short summary of the differences here. While there are technical differences, both are fine movements and will serve you well.
Regarding the 24 hour hand... it's a nice feature, but in practice it isn't really needed. If you aren't sure if you are looking at AM or PM, just advance the time until the date indications change and keep going til 6 AM (a "safe" time). Then make all the necessary calendar adjustments, and advance the hands to the current time and you are good to go. If the watch had stopped today and you aren't sure if it is AM or PM, just set the time. The watch will let you know if you are fast or slow by 12 hrs :) To me, the moonphase is a more attractive option (although not necessarily any more useful...).
Size does matter - but it depends on your own personal preferences. My 5056P is 37 mm and at first felt a bit small - but I much preferred the roman numerals and dial to that of the 39 mm 5146. Plus in terms of value (not investment!) I was able to get the platinum 5056 for about the same price as the white gold 5146 (preowned market). So for me, the 5056P was the more sensible choice, and I do love that watch!
- zhege