If you actually do a quick search, you find that the Cobalt heater is 6.5" x 2 1/8" x 4/5". The Archaea is 4.5" x 1/2" - and its controls are outside the tank.
The Cobalt is twice the price for a reason. It's a better piece of equipment backed up with a warranty. The Archaea is some re-branded stuff that you can get on eBay and other auction sites if you search hard enough.
I'd go with the Cobalt because it's less obtrusive and easier to disguise in a tank with a black background.