Help choosing best danfoss compressor for single stage

[Stelios]

I'm new at Vapor Phase change cooling and right now trying to figure out what will best suit my needs.
My main concern is the compressor . The only sure thing is that i'm going to buy one of these :

http://compressors.danfoss.com/produ...z/s-series.htm

The candidates are : sc12cl , sc15cl , sc18cl and sc21cl

AFAIK the sc21cl is 3/4HP , the sc18cl is 5/8HP and the sc15cl is 1/2HP ...

Here's my questions :

I'm aiming at a full load of approx. 200-250W (Intel Prescott CPU) .

1) I want to calculate the efficiency of each compressor at 300W heatload . How's this possible ? Let's say for the sc18cl .
Are we looking at cecomaf or the ashrae numbers ? For 300W it can make approx -40celsius . But how much power consumption will it have ? The "Power consumption" and "current consumption" don't match ... What am i missing here ????

2) Looking at the specs , i see that Is that up to the sc12cl the current consumption scales up pretty good . From the sc15cl the current consumption scales up fast . Does this make the sc12cl the most economic compressor from the others when operating with the specific heat load ???

3) Let's see this from the hardcore overclockers point of view . What are the benefits from choosing the sc21cl from sc18cl ? It's about 3 degrees cooler and it has less power consumption . I'm i right ?

4) Can someone explain to me what the COP means ?? I understand that it is capacity/power consumption . Is this the efficiency ??? So , are we looking for the higher possible COP with our heat load ?

Thanks !