I think Hyperdimension Neptunia some people got turned off by Ar Tonlico Qoga battle system at same time some people hate Trinity Universe and scared of this. either or I'm buying both but like Hyperdimension Neptunia bit more.
The only reason i want neptunia more is because AT3 doesn't have a playable akane. I hate them so much for that...to top it off the game doesn't even have real costumes just alternate personalities. I'm getting both but AT3 just seems halved ass'd when I look on it.
I'd have to say Ar Tonelico...it already has an established fan base, which gives it a one up on sales...though I'm sure quite a few are buying both, more people new to the series will pick up AT, because there are males (as shallow as it sounds)