I bought a M1A1. It's the third tank I've bought. I have a terrible time filling tanks without leaking and getting the end caps back on........No else seems to have the difficulty that I have with them. I bought the M1A1 because of the filling port which seems to work fairly well. I still get some air pressure leaking problems, but it's manageable.
The M1A1 has more slippage then the other tanks I bought, but, again, it's manageable.
I preferred the looks of the Lil Mama SS end caps tank, but found it impossible for me to fill or put in new cartos. This tank had the tightest end caps of any that I tried.
The Chibi tank was OK, but you have to be very careful when filling with a syringe not to nick the delrin top which will cause air pressure leaking problems.
Even though I'm not crazy about the looks of the port on the M1A1, it's been the easiest to use.