So Contemplating a 4K upgrade, what on earth is the main difference in these two boxes? Specifically what is it that justify s the £100 price increase for the ultimo?

All I can see is that the ultimo has

one more slot for tuners
has slightly more memory but a slower CPU
a bigger front screen.
RCA audio outputs
2 x smartcard readers (vs 1 on the duo)

Am I missing anything in terms of 4K support or decoding capabilities?

Right now the Duo seems like better value for money

Thanks in advance