Personally, I'd go with an AquaClear 50 over the Penguins. I have an AC50 on my 29gal and it is awesome. I had a Penguin 200 on the same tank and the motor burned out overnight. Also, the AquaClear has interchangable media (sponge, BioMax rings, activated carbon, Ammonia Remover, etc), whereas with the Penguin and Emperor, you're stuck with the filter cartridge only.
The AC 50 is $40 at Petsmart. The Penguin 350 is $50 at Petsmart. The Emperor 400 is $75 at Petsmart. Heck, you could even go with the AC 70 for $60.