F2p. You'll get the game mode, but riot will try to sell things a long with this probably, like themed skins. Everyone will be gushing over skarner and his epic skin to go with the crystal scar, and you'll put up with it every game. Riot wants this. You provide three things for them by being f2p.
1) Free advertisement
2) Sustained gameplay for paying customers (p2p is great, p2p with no players is bad.)
3) Constant temptation to make purchases.
Taking this into account, you can decide logically that the game mode will be available to everyone.