F2P means Free to Play (very restricted on what you could do). F2P is absolutely not a trial or demo version of the game. Free to play variant is simpler - players can still level up their abilities, but it's a lot harder and bored. Also there's a trade limitation to players that haven't been a member before.
Free-to-play is a fantastic start for novices as they will not be doing anything except figuring out how to play that is far simpler to perform in free-to-play. Also, since there are not as many quests to select from, Beginners will have the ability to make faster decisions, therefore enhancing their character faster.
P2P means Pay to Play (what's unlocked). The largest free to play benefit is ten additional skills. In contrast, Pay to play variant players have 10+ RuneScape skills. There's ability to buy ability capes when reaching level 99 (and 120 in Dungeoneering's case). Higher capabilities in different skills. Much bigger explorable area.