1.MSI MAG Z490 Tomahawk 10th Gen ATX Motherboard
VS
2.MSI Z490-A PRO 10th Gen Intel ATX Motherboard
which one is better in performance
if I use i7 10700k on both ...
which will perform better and how much is the difference in performance
When you are dealing with 2 motherboards from the same manufacturer using the same chipset, unless one is just obviously using crappy components then you can safely expect relatively equal overall performance.
MSI is a high quality manufacturer so you can GENERALLY feel pretty safe that two different boards with the same chipset with be about the same overall with the difference just laying in bells and whistles beyond raw performance being about the same.
That said, the Tomahawk is a more premium board, but not MSI's top line and might be thought of like the TUF series from Asus, middle of the road. The Z490-A Pro is a more basic board and should provide roughly same performance as the Tomahawk with less overclocking ability and less bells and whistles (extra features).
If you can budget in the Tomahawk, I recommend it as it is a nice board and reviews speak fondly of it. It's not head and shoulders above the pack, nor is it behind. Solid middle ground board with some goodies. As I said before, a lot like the Asus TUF overall.
Edit: HOWEVER, to clarify what I said above, budgeting in the Tomahawk should not come at the expense of some other intended or needed component running at the specs you are already targeting. If it comes down to get the Tomahawk and less or slower ram, or getting the Pro and the faster ram or maybe bigger M.2 SSD, do that instead...
