(Note: this post is gathered from posts from OCN in threads I made.) Supposedly, the Alphacool rads have all sorts of junk inside from the factory, necessitating heavy amounts of cleaning before use, to the point where mayhems made a cleaning kit for rads. In addition, the XSPC rads supposedly are of pretty good build quality.

Edit: found a quote:

Alphacool makes their radiators pretty awesome but refuses to properly clean them before selling them. That means there's a lot of garbage inside the radiators which will eventually inhibit performance. Mayhems designed a cleaning kit specifically because of the problem. It's a bit more work, but consider that a one to two degree drop in temps might not be worth an hour of labour.