- I need CPU power. I read that it is possible to put a i7-3940XM in a T530. Because of coreboot it should also possible to put it in a T520?
Yes, with Coreboot you can put an Ivy Bridge CPU into a xx20 series ThinkPads.
- I would like to use an eGPU. Any differences between these two models?
Not really, especially with coreboot. I'm glad to report that my RX 580 eGPU works well with my T430 running Coreboot - just make sure you enable ONBOARD_VGA_IS_PRIMARY in your config, otherwise your integrated graphics will be disabled and you'll lose eGPU hotplug.
- With which model it is easier to install coreboot on it?
No practical difference, the building and flashing procedure is the same.
- What are the general pros and cons of a T520/T530, especially with coreboot?
T530 will not accept non-Lenovo batteries unlesh you flash a patched EC firmware, and that can only be done while you're still running stock UEFI. T530 has USB 3.0, T520 has a keyboard that some consider superior. Otherwise the machines are extremely similar.