standpipe advise

[fishmaster]

Ok, I'm in the process of building my 120g. I will be running a PS4 return pump at about 5' of head. I plan on using a variation of the stockman standpipe-the gurgle buster. My question is...for that amount of flow, should I be running two 1" standpipes, one 1.5" or two 1.5"? The main object here is SILENCE!!! Any other suggestions would be much appreciated.
Thanks in advance,
Shaun.