I didn't think Bowser's Inside Story was all that great. It was decent (and definitely a step up from Partners in Time), but at times it got pretty boring due to a lack of enemy variety. Most areas only have 2-3 different enemy types. I could check my playtime later, but I think it was about on par lengthwise to the other games in the series.
I'd recommend Professor Layton. I don't have it yet, but if you enjoy Layton, you'll enjoy the game. It's supposed to have daily download puzzles over wifi for a year. So that's over 300 puzzles not including the ones in the story as well.
Edit: I should mention that it probably doesn't matter if you've missed previous Layton games, as this one is supposed to be a prequel to the others.