I personally love my Tunze Nano on my 15g IM AIO.
One factor that I haven't seen discussed with most ATOs is noise level. Because I've got a tank in my bedroom I prefer to have an optical ato that's got a quieter pump than the Tunze nano, but I will say the Tunze float sensor is more accurate than the optical sensors I've used with "cheaper" ATOs.
Anecdotally, of the brands I've used
Optical: MostAqua opitcal sensor at 3 years old holding up as good as day one, no pump issues either; AutoAqua is the least accurate of the brands I own, somehow the optical sensor "flipped" itself and it fills only to the bottom sensor once it cannot read the water level. Magtool optical sensor is so so? Its calibration is the worst of them all and often overfills unless your reservoir line is feeding directly next to to the optical sensor, otherwise it seems to overfill the 10g tank its on.
Float:
Tunze nano is awesome, it's just super frickin loud when it actuates. For that reason alone, it does not get to enter my bedroom and only exists in the living room/dining room.
I have a Tunze 3155 I've not used but I can imagine it SHOULD function similarly to the nano, just for tanks larger than I own.