Actually, the PS3Slim is pretty quiet.
I think some of us have different standards on what is quiet and what isn't. At regular listening distance, the Mac mini is absolutely silent (and I have SMC Fan Control running at +500rpm to keep it running cooler). My slim PS3, however, has an audible whine to it that I can't STAND at low volume levels.
My Mac mini 2009 model (2Ghz, 9400m, 4GB DDR3, 320GB 7200rpm + 8TB RAID enclosures via FW800) is absolutely perfect for an HTPC. I run movies/TV shows off it all day long with Plex. I wouldn't switch to the PS3 for media, ever.