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the 900 is the better case, try to see if you can cancel the order, as the 900 has better cooling, and the sound proofing is simular, but the thing that really ameks it better is that the way the p180 works to keep the computer cool is by seperating the psu and hdd from the rest of the case, which is nice for quietness, but takes up a lot of the room it potentially has for upgrading cooling and such
The P180 is easily the quieter case. The 900 may have slightly better cooling, but the noise factor is not even close. The open side and front vents of the 900 allow too much noise to pass directly out of the case.
Replace the 120mm Antec fans with Nexus or low rpm Scythe fans and use a SeaSonic/Corsair PSU, and the P180 is even quieter.