---

4790K
Pros:
Higher Base Clock Speed (4GHz)
Lower TDP (88W)
Cheaper (£252)
Cons:
Less Cores/Threads (4/8)

---

5820K
Pros:
More Cores/Threads (6/12)
Utilises the X99 chipset platform
Cons:
Lower Base Clock Speed (3.3GHz)
Higher TDP (140W)
More Expensive (£303.80)

---

The 5820K doesn't have an iGPU opposed to the 4790K which dies, but having processors like these you would be expected to have a discrete GPU anyway.

It may look like the 4790K is the favourable CPU, but the 50% more cores and threads in the 5820K will make a big impact if they are being used for example in heavy rendering and editing tasks. The Clock Speed and TDP difference also doesn't make a huge impact, as you can overclock either of these chips and you would need a pretty beefy PSU anyway so the TDP shouldn't affect the build.

Go with X99 if you

1. need more than eight CPU threads
2. need more than 16 PCIe lanes
3. can make use of more memory bandwidth
4. have money to burn

It's very unlikely any of these apply to you.

Comparing clock speeds of chips with different architectures is meaningless.