The dynamics of the aircraft acquisition market are constantly changing, so what was true a couple of years ago may well not be now. Several high-profile operators have gone with the Q400 over the ATR, what does that tell you? Particularly Flybe in the UK, amongst others. The Q also has a much nicer flight deck IMHO.
Having said that, they are both horrible, very lightly built and quite fragile. I doubt many will be around in 20 years.
The Q400 has been around since 2000 (so 11 years more or less) and has 406 orders, the ATR has been around since 1988 (23 years) and has 436 orders... so the question of which is more popular is pretty easy to answer.
The Q400 is also a lot faster (360kts max cruise for the Q, 276 kts max cruise for the ATR), goes higher (FL270 as opposed to FL250) and has a greater range (715nm for the ATR and 1567nm for the Q). If you believe Wikipedia, anyway...