Groupset or Wheelset?

I am lucky enough to be getting an £800 fitness allowance from work, which naturally means bike stuff. I don't need kit and can only buy for myself from a registered company (for expenses). I currently have a Canyon MTB, Cinelli Gazzetta, Mercian King of Mercia and Cannondale Supersix Evo, so don't have space for another entire bike.

Was thinking of upgrading the Cannondale (my best bike), which currently has Campagnolo Zonda Wheels and 105 5700 GS. So, big question is: New GS or WS? or any other suggestions?

Since you really don't need another bike :-), how about spending the money on a training camp somewhere warm?

Quicker wheels will make a bigger difference to the ride. you'll realy feel acceleration improve if you save weight in the rims.

11s 105, it is lightyears nicer than 10s stuff, and 11s ultegra isn't really worth the extra premium over 105.

You might then need new wheels, but you should have 500 or so left from your budget to buy some fancy wheels.

TLDR, buy both

Powermeter

Quicker wheels will make a bigger difference to the ride. you'll realy feel acceleration improve if you save weight in the rims.

Yes: if by "quicker" you meant "more aerodynamic".

But lighter rims have a negligible effect in themselves.

http://alex-cycle.blogspot.co.uk/2013/02/the-sum-of-parts.html

http://alex-cycle.blogspot.co.uk/2014/12/the-sum-of-parts-ii.html

If i have extra cash, I'll spend it on a professional bike fit if you havent had one yet.. Having one is such a good investment imo.

105 11s is so good right now for the price

Shimano 105 11 speed and new wheels, as you wouldn't be able to use the old ones

Good combo would recommend.

But lighter rims have a negligible effect in themselves.>

Really surprised to hear this. when I went from a £200 up to a £600 wheelset, without aero rims, the effect on acceleration and climbing power was dramatic. Obviously there's bearing quality and stiffness as well, but it seems odd the minimal inertia in a light rim has little effect on how easy it is to get it moving. I'm sure @mdcc_tester has more knowledge on this.

I just read the article you posted, and the scientific experiments it links to that prove rotational inertia is negligible in acceleration, and my conclusion is that science is stupid and I have magic wheels.

it seems odd the minimal inertia in a light rim has little effect on how easy it is to get it moving. I'm sure @mdcc_tester has more knowledge on this.

No more than anybody with a basic grasp of mechanics. If you add up all the inertia in a bike, you'll find that the rims and tyres make up a very small proportion of the total. Inertia only matters at all when you're accelerating, which is something cyclists do as little as possible, and rather slowly when they do, so the total net (allowing for the fact that you recover the energy used to accelerate as free propulsion when you coast) energy expenditure from inertial loads is an even smaller proportion of a total which is dominated by aerodynamic drag and rolling resistance.

when I went from a £200 up to a £600 wheelset, without aero rims, the effect on acceleration and climbing power was dramatic

It was only dramatic in the sense that it was a fictional narrative, first related to you by the wheel salesman and then repeated to yourself as post hoc justification for spending all that money.