Fantasy Life will have more for you to do, to be honest. I love HHD, but it's not something that can be played hours on end because it'll get boring. All you do is design houses.
Fantasy Life has more than 100 hours worth of gameplay, especially if you buy the DLC for it.
I absolutely love both, but if you had to choose, I'd choose Fantasy Life. It provides something new for you to play, plus it'll keep you busy for a long time.
Fantasy Life is more entertaining in the long run, in my opinion. I've put in over 70 hours in just the last couple of weeks after picking it back up again and resetting completely, lol. I've also done more in this new file than I ever did in my old one so I can't picture resetting ever again. Good thing there's the other two save slots to keep me entertained if I want to replay it.
HHD, at least if you liked the designing and decorating about NL. I'd guess both are about as much money grabs but I'm not too much into grinding rpg-ish things anymore.
they're so different it's hard to say. although i'd say i slightly preferred HHD i have way more hours in fantasy life, there's just so much to do in the game so it's probably worth the money more than HHD is