As for front/rear speeds, even Bitmain can't seem to make up their mind. As a power systems designer, personally I'd put the faster fan on the intake side to provide higher pressure airflow in to overcome resistance from the turbulence all the little heat sinks make. What might change is the noise.
Thanks for the info! Good to know. I know they have different fans, but I don't know yet which one is faster (can't find the specs for their fans - mine have NIDEC UltraFlo W12E12BS11B5-57 on the front and NIDEC UltraFlo W12E12BS11B5-07 on the back) and the UI doesn't tell you which one is front and which one is back.