The SCWD will reduce your output considerably, but it is a lot cheaper than the sea swirl. I'd go with the sea swirl if you can budget it in, but I can't help you with the size because I still haven't been able to budget it in.
Agreed with Pit Pat. Also, SCWD tend to get their gears gummed up and need to get replaced once or twice a year.
You should also consider an oceansmotions squirt from www.oceansmotions.com.
Seio is a very different device from any the others mentioned so far. A seio will give you constant current in one direction - not a bad thing. The others give you current from different directions.
The sea swirl and SCWD do different things. I had SCWD and the worked fine, but as others mentioned they do cut your flow by quite a bit. I have a ocenmotions squirt for my new tank. Don't know how it performs yet, but the workman ship was first rate.
I'm a fan of swirls... while their motors may die on you, they don't require the cleaning a SCWD does as the water itself isn't being redirected inside the device. The only negative to them is that they sit right on top of the tank and thats trouble for some people who have close fitting hoods.