I have both.
They are both great and whichever you choose you will be very happy with.
If you don't need 3 tuners, then just get the Duo, if you think the extra tuner will be useful then get the Ultimo.
In use there is little to choose between them, the Ultimo is faster (more memory, faster cpu etc), and it has that lovely front panel vfd display too which makes it worth the extra money imo and that is why I bought 1.
If you do plan to go with the Duo, then as suggested on the other site you asked this question on, you are better off waiting for the 2nd edition Duo to hit the shelves as it sounds like 1 of the best just got better.