I was looking at the Sonata and the Tsunami myself. I ended up choosing the sonata because of cost, mostly. The Tsunami would have been a bit more with a decent PSU and a fanbus. The tsunami is a bit louder than the sonata but if you run it off of a fanbus, you can contol the noise. The sonata, OTOH, is controlled by the psu temp. Mine never gets up to high enough speed to be loud. I'm running a P4 3.0c (stock speeds), BFG 6800 OC (all pipelines anabled) with an Arctic Cooling VGA silencer, 1 gig of crucial pc3200 ram, a seagate 7200 rpm hd, and a dvd-rom. My cpu stays fairly cool (Thermalright SP94 and enermax 92mm fan at 2200 rpm) at idle but gets a bit hot under load (~50) for my tastes. I would have liked for the fans to have been more responsive to the cpu temps vs the psu temps.
Anyway, if you're going to be overclocking or running something really heavy, the Tsunami or P160 may be a better choice as you can more easily control the fan speeds.
I am also a bit dissappointed in the paint job on the Antec. It looks nice but scratches easily. My only other complaint is the front fan is mounted behind the hard drives.
It IS a very quiet case, though, and it has a lot going for it. I'm very happy with it overall.