I used to be a die hard Seal supporter, I ran one, was a distributor, and apart from a couple quirks, they were by far one of the best out there. That was years ago, I got out of selling them, and just ran my 600D and my 600C, hundreds of thousands of feet of laminate later and I still stand by those units.
Lately I've been coming into contact with the Ultra series, and every single last person that I know of, has at least one major issue with them. Personally I wouldn't take one if it was given to me. Their "quirks" prevent them from running even remotely correctly.
That being said, I'm not a big fan of laminators with small rollers, but still the disadvantages far outweigh the advantages in my opinion. The 600 series is still a solid laminator, the frame alone let's you know it can handle serious business. GBC, AGL, and royal sovereign are all solid contenders.
From past experience, apart from having to get the tensioner arm replaced ( because I had #3 off the assembly line) I would take a 600 over most other laminators out there. ( I do like the way other laminators remove the need for an air compressor tho)