Acnl for sure! I got the HHD 3ds when it came out and I've only played HHD for a bit. Acnl is something that I can keep coming back to and it keeps me entertained for hours on end tbh.
HHD is a good way to pass 20 minutes or to get a good reference for what all the villagers look like in game, or sometimes an okay creative outlet, but I really think it's no contest in comparison to ACNL.
ACNL. I don't even own HHD yet, and after seeing a lot of the reviews on it, I might wait a little longer before I spend all that money on a game I might not enjoy.
I like HHD but it's a completely different concept and I don't think it can be compared. NL is a "forever game" to always come back to and that ever changes and I play it every day. HHD is a cute little game that I play sometimes when I feel like designing homes, haha. If I could only choose one it would OF COURSE be NL.
I absolutely love HHD but I play on and off. I'll play all day for a week and stop for like 2 months. ACNL is a daily thing, and there's mroe to do. (though it's very frustrating designing houses on ACNL after playing HHD with all its possibilites )