Z490 boards are an incremental upgrade to support Intel’s new socket design the LGA 1200. Its main advantages over its aging sibling is faster memory support, optional 2.5GB ethernet, and wireless Wifi 6 standard protocol.
The new socket is the big deal here as it means to get close to the 10th gen chips you need a new mobo too. The package of the actual chips themselves is the same meaning that existing coolers will fit just fine, but just don’t try and ram a 10th gen CPU into a 9th gen socket and vice versa.
Besides that, the chipsets are remarkably similar between Z390 and Z490 but it’s the access to the new socket that is going to cost you.