I’m currently using a NETGEAR Orbi mesh system and it works great. Assuming an extender works as advertised (and it sounds like you’re having poor results with that) the big difference that will be noticeable is that it’s going to look like two distinct WiFi access points. So you’ll be on the “MyAccessPoint” WiFi network close to the main router and the “MyAccessPoint_extension” (or something like that) WiFi network close to the extender.
That can actually be quite irritating. In my experience from a few years back with mobile devices the switch to a WiFi network with the stronger signal is not a seamless or quick action. It may hang onto the weaker original network even when you’re next to the extender. Which defeats the purpose or requires you to manually switch. Either way, annoying.
Mesh systems basically look like one big, strong WiFi network. As said they have a separate dedicated wireless backhaul between the mesh access points that is unlikely to interfere with your client device connections. I find I can go anywhere in our house now (we have three access points including the main one) and the connection and speed is extremely robust.
I think mesh is the clear superior solution if price isn’t a consideration. But they are significantly more expensive (I think I paid $300 or something close to it for my three access point system).