Seeing as how I just ordered a new skimmer for my 180 gallon set-up, I thought I would explain how I pick out skimmers.
Here are the three simple rules of thumb that I used to pick out a needle wheel based skimmer. These are not hard fast rules, just my own personal guidelines that I have decided to use for myself based on what I have read. Nothing really too scientific about this, just my first hand experiences combined with what I have read. You have to also keep in mind; some manufacture stated capacities are based on very specific test conditions which are not always reproducible in the average set-up resulting in a stated rating that may not always be usable or 100% applicable to real life uses. That is why I like to pick out skimmers based on the below three simple guidelines:

A) Look at the Airflow Ratings.
The reason this is important is to try to gauge if the pump will be capable to produce a lot of fine bubbles. The higher the air rating, the better the potential performance. Although a lot of skimmers will be set to use about 10% to 25% of the maximum air intake settings on the skimmer (in liters per hour), this will still give you a somewhat usable guideline even though manufactures will state the maximum un restricted air flow the pump is capable of. Higher l/hr ratings can produce larger amounts of bubbles as well as smaller sized bubbles which are two key factors for a skimmer to work efficiently. For my set-ups, I have had great success by picking a skimmer with a L/hr that is between 5 to 10 times the total water volume of my set-ups (display tank + sump water volume)

B) Look at the water flow rating (in gallons per hour).

This is the amount of water the skimmer will move. I look at this for two reasons. The first is to make sure the skimmer will be able create strong and turbulent flow to utilize the bubbles produced and push the crud up into the skimmer cup. The second is to make sure the skimmer will be able to skim anywhere’s from 4 to 6 times the total water volume in the set-up. However, as the air pulled into the skimmer will offset some of the water, you must substrate at least 25% of the flow rating to get a more realistic number from the manufacture’s stated flow rate. This should give you the minimum water flow you will get out of that skimmer (worse case situation) based on average air/water mixtures. For example, if the manufacture states the skimmer pump has a flow rate of 1000 gph, you should use a flow rate of 750 gph as that is closer to the actual amount of water it will move once air has been mixed into the water.

C) Look for a good quality skimmer with a good quality skimmer pump.

This will also depend on you budget to purchase a skimmer. I personally pay more attention to the skimmer pump than the brand of skimmer. Pumps such as: Sicce, Red Dragon, Diablo, and Rock rank amount the better quality skimmer pumps, but there are a lot more out there. Take the time to research the brand and read a lot of customer reviews to get some firsthand feedback.

When I researched the skimmer I just ordered for my 180 gallon set-up (total of 250 gallon water volume), I knew I wanted a skimmer with 1000 to 1500 GPH and 1250 to 2500 L/hr. I also wanted a very good quality brand as I have the budget that will allow for that. I looked at the below brands.

1) Bubble King, Model: Supermarine 250.
Pros: performance and the red dragon pump (2000 L/hr air intake, 1000 gph water flow), Cons: It’s freakishly large and expensive, and no local dealers were I live. I decided against this one due to cost and no local dealers.

2) Vertex, Model: Alpha Cone 250,
Pros: performance, red dragon pump (1800 L/hr air intake, 1000 gph), Cons: cost & size. I decided against this one due to cost and large foot print required my not work the best in my sump.

3) Reef Octopus, Model Diablo DCS250-INT,
Pros: affordable, diablo DC pump (2400 L/hr air intake, 750 gph), Cons: size, no dealers where I live, lower GPH than I wanted. I decided against this one due to cost and no local dealers and low GPH.

4) Bubble Magus, Model: Curve9 cone skimmer,
Pros: cost and size, Cons: Rock SP4000 pump (only 900 L/hr air intake), reported to be a noisy skimmer. I decided against this one due to the low volume air intake and the many firsthand reports of higher than expected noise levels. I was also unable to find GPH info from a creditable source.

5) CSC, model 450.
This skimmer is a Skimz SK251 body upgraded with two Sicce PSK 1200 pumps. Pros: two Sicce pump (1680 L/hr air intake, 2090 gph in total), and the size / small footprint. Cons: Limited distribution outside of Canada. I decided to get this skimmer due to: it being the second cheapest of all the skimmers that I looked at, it has two Sicce pumps with good air intake, local dealership support, meets both of my L/hr and GPH requirements, and the first hand reported performance.

A little more info on the skimmer I just ordered