I bought the beginner bundle with RP right out of the gates, which landed me with a handful of champions that I don't like to play and blown RP on champions that I could have earned IP for far too early. Instant regret.
Honestly, I don't think new players--or any players, for that matter--should be buying champions with RP. Buying with IP gives you time to study the last champion you bought and spend quality time learning them rather than mindlessly fleshing out your champion pool.
The first champion I bought was Pantheon, who I completely failed with because he had a big red shield that led me to believe that he would be incredibly tanky. Obviously this is not the case, and I don't think I played more than two games with him before blowing some RP on Wukong because I had decided that I made a bum purchase. If i had waited until I earned the money, I would have spent the time to really understand the champion--and really, would have understood assassins as a whole--much better than i did. New players are just too focused on finding their 'main' and not focused enough on learning the game and the roles.
Also, nobody explains AD/AP. So I played Ravenous Hydra Nunu. Every time. And Trinity Force ofc, because it was the most expensive at the time and therefore obviously the strongest item in the game. So everybody got a trinity no matter who I played.