I haven't had a chance to play so I can't say which I like more, but part of the reason I like ARAB over ARAM is that because it's so small and the respawn timers are so low, the playstyle is suited more to constant teamfights rather than whichever team has better poke/sustain wins.
ARAB is about which players are better, whereas a lot of ARAM is about which team has better champs. Other than Heimer, there are no real "auto-win" champs in ARAB, while in ARAM there are champs that just dominate the game (Nidalee, Blitz, Ziggs, etc.) regardless of the overall skill level of the entire team.
This is of course not to say that all ARAMs are decided at champ select, or that all ARABs start totally evenly, but it definitely affects the playstyle and I hope the Proving Grounds will lean closer to the ARAB side of things.