so 55$ in the difference but 20 % faster i think it would be a better upgrade for 55$ considering a 2060 over a 1660 ti is like $130 in the difference but only about 18% faster. and considering im only using a 32" 1080p 60 hz monitor i think the 1660ti would be more than powerful for years to come.Price difference between a 250 and 500, could be up to $70USD. And being that you plan on putting games on a separate ssd, using the boot drive for larger more played games, won't net you a huge difference in loading times. It would IMO be negligible. That could be money used elsewhere.
Odd your seeing a speed difference between a 250 and 500 of the same line, my guess is difference in the memory controller.