Which overflow box and pump?

I'm going to be adding a 20 gallon sump to my 55 gallon reef tank that's been up and running for 3 1/2 years. Due to my stand, the sump will have to sit next to my tank. I'm looking at various pump and overflow box options. First of all, what size overflow box would you recommend? Also, what size pump? Second, I've been looking at Eshopps overflow boxes, but am interested in how the CPR boxes perform. With the CPR box I am limited on which box I can get by the space between my wall and tank, so I would only be able to use the CPR box that's rated at 300 gph.

I've been kind of leaning towards the Eshopps PF-800 with the Mag 950... is that pump too strong though? Also, do I need that large of an overflow? I've been concerned about the u-tube and getting air bubbles in there and losing siphon, that's why I was considering the CPR box with the Aqua Lifter pump. Anyone experience this problem before or know how to prevent this from happening? Thanks!
I have no clue about the CPR overflows, but I love my Lifereef overflow, it doesn't require an aqualifter, and is guaranteed not to lose siphon in a power outage.
I had never heard of Lifereef before. I'm interested. How long have you had it? What size did you get and what pump are you running with it? Also, SeaBee,wha size Eshoppes do you have and what pump are you running with it? How long have you had it?
I've had my lifereef for about a month now, it's paired up with a Magnum 5 pump. That overflow will handle i believe 750GPH. Plus Jeff over there is Great! He's helped me with all my question's I've had with their overflows. I also think they are all hand-made.