Whatever you end up doing with that machine, do not forget to upgrade each and every firmware for which updates are available (Mandatory; BIOS/UEFI, graphics card, storage), Max Out the RAM (mandatory), put a SDD and maybe a better graphics card (if budget allows). After the BIOS updates, enable Hyperthreading on the P4. Since the machines are so old, go for 32Bit OSs (as this will improve your chances to get drivers)
If you can put 2GB RAM (or more) on both machines (and perhaps a small SSD), the best upgrade path is Windows 10, or Windows 8.1 + ClassicShell (depending of course on Processor Instruction Support and Graphics card support).
But Your Question was Specific to Liux, so, if you want a desktop, and the machines have reasonable good Specs, go for Ubuntu straight ahead. Is better to learn a standard distro, as both the general and specific knowledge that you gain will be transferable/useable in a work environment.
If your machine has somewhat weaklish specs, go for CrunchBang!++ . At least you get a functional distro...
If you want the opportunity to learn your way around a server (in particular with the desktop), go for CentOS 6.3 (the last supported version with 32 bits support).
Best of luck.