The XSPC RX360 and Hardware Labs Black Ice SR-1 360 are two of the best radiators for low speed fans and they are very close in terms of performance
I was leaning towards the SR-1 because the paint finish on them was supposed to better than the RX360. The SR-1 has a matte finish and RX360 a glossy shiny finish but I believe that has changed with the RX360 rev2 which is supposed to have the same finish as the SR-1 nowadays Something to keep in mind when looking at pictures online since a lot of shops still use the older picture.
From my research a lot of people ended up going for the SR-1 because of the superior paint finish, but now with the rev2 I think the choice has become much harder.
Ultimately I decided to get the SR-1 because its more "stealthy" than the RX360 since it has no logo anywhere. It looks nice and discrete in my case. The XSPC one on the other hand has their big fat logo stamped on the side. Don't know if that would bother you or not.
Here are some helpful links so you can decide on your own:
Some interesting graphs here comparing various rads with different fan speeds:
Also lookup unboxing videos on youtube to get an idea what they look likeEdited by vvoc - 3/24/12 at 10:21pm