Well, specifically for this addon, if you want to only find players hours when they're offline/online, I would go with SteamID32, because that's easier to find. Honestly though, where are you trying to go with this addon right now? Do you just want to use this addon within your server, and keep all storing information on Garry's Mod's SQL database, or are you planning on expanding this addon to make it available for any servers you may have?
@Aaron113, how much more memory could using Steam32 bit even take? Maybe I might be doing things wrong, and could improve on my servers status by updating things.