Question regarding upgrading wheels- cassette too?

Hi all,
I have a Specialized Tarmac SL4 Sport and about to upgrade the Axis 1.0 wheels to Mavic Ksyrium Elite S wheel set.
The Tarmac has a Tiagra 10 speed cassette on the existing wheels.
Is it a good idea to upgrade the cassette at the same time?
Also, I was thinking of going for the Dura-Ace 7800 10 speed (£94) or the Ultergra 6600 10 speed (£50).
Any reason why I shouldn't do this?
Is the Dura-Ace just a bit lighter?
They are 173g vs 235g.
I guess worth is relative here but if you were in my position which one would you go for, if any?
I was plan on upgrading many of the Tiagra and 105 components to Ultegra/Dura-Ace over time.
I have a Specialized Tarmac SL4 Sport and about to upgrade the Axis 1.0 wheels to Mavic Ksyrium Elite S wheel set.
The Tarmac has a Tiagra 10 speed cassette on the existing wheels.
Is it a good idea to upgrade the cassette at the same time?
Also, I was thinking of going for the Dura-Ace 7800 10 speed (£94) or the Ultergra 6600 10 speed (£50).
Any reason why I shouldn't do this?
Is the Dura-Ace just a bit lighter?
They are 173g vs 235g.
I guess worth is relative here but if you were in my position which one would you go for, if any?
I was plan on upgrading many of the Tiagra and 105 components to Ultegra/Dura-Ace over time.
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Ok the Ultegra is lighter but that was about it.
D
Perhaps I'll hold off upgrading the cassette for now and wait until it is worn.
Cheers for that- yes a lot cheaper.
For the reason of cost/benefit ratio, or something else?
I would imagine cost benefit.
D
Thanks for the help folks.