Oh, one thing about the Bullets: If they haven't changed the design recently, you may have a problem fitting even the Bullet 1 in a 6" wide space. The base dimensions actually are a lot wider when you have the effluent ball valve installed. You can probably have PM custom build you one with the hole drilled on the narrow side.
If it's just the reaction chamber's diameter that is the issue, then ignore my previous comment
Looking at the pictures of the PM the base is only about a 1/4" wider than the reaction chamber. Which would still fit(even if it was a 1/2" more it would fit). The reaction chamber is 5.25" and I have up to 6". As far as the ball valve goes, I could screw it in after it is in the sump.
There's actually a big gate valve that will stick out of the side of the PM Bullet, so it'll add another 3-4" to the total width of the base. The pictures show the PM without the gate valve threaded on. I'm guessing PM can drill the hole for the gate valve on the narrow side so as to keep the unit only 5" wide (but it'll be longer).
The Bullet 1 will do better then your Berlin Ideally you want something bigger but it sounds like you are very tight for space.
I have enough room in the sump, the problem I'll have is getting a skimmer into the sump itself. The base on the berlin(6" sqauer base) just fits through the opening. I have even less clearance from the sides due to the added braces AGA put on their stands under the tank.
I'd love to get the bullet 2 or a euro reef but I do not think they'll fit.