I3 8100 or ryzen 3 1300x?
I have Ryzen 1300x. Is a very good cpu. With i3 8100 you will spend more money cause there aren't cheap mobo.
Specifying the form factor of motherboard you want plays a big role into the cost difference of the platforms. If it's just normal ATX, you'll save maybe $20-30 on a cheapo AMD motherboard, but you'd give up a little performance with a weaker CPU compared to the 8100.
I can get a Gigabyte H110M for 60 EUR should I Do that or buy a ryzen 3 1300x?
That ain't compatible with the i3 8100, so ye, you should go with the R3 1200/1300X, unless you want to wait till next year when budget Coffee Lake Mobos will be released.