Yellow has a few different moves allowable and (if you have access to a cheat cartridge or Pokemon Stadium) a bonus minigame. Aside from that there's a few extra NPC trainers dotted around to follow the Anime a little better. However, it lacks an easily accessable Missingno glitch.
Red & Blue are very much the same game, although if you're not glitching / trading, the overall "Base Stats" of the Pokemon exclusive to Blue are a little better than those exclusive to Red. Nothing most people would notice however. The only real differences between Red & Blue are the default colour schemes on a GBC / GBA, and the Pokemon Gengar is fighting during the intro sequence. --- Where logic won't work - I will PAL Gamers unite - http://www.gamefaqs.com/boards/277-pal-gaming
I'd say Yellow is more difficult as the enemy Pokemon often have better movesets. --- Capt. Basch fon Ronsenburg, for Incitement of Sedition and the Assassination of H.R.M. King Raminas, has been found Guilty of High Treason and put to his Death.