You are gaining way more then an OS upgrade- you are gaining app support, firmware upgrades (HTC gave the Titan no support when WP7.5 was new, they sure aren't gonna do anything now), and improved processor performance to run these upgrades.
Obviously, this boils down to your Titan and your desire. If you are interested or if your T1 is showing signs of age, pretty much any Nokia will do the trick. As a complete side note, the L625 will have the same size screen as your T1 if you can find one in your price range. Otherwise, those L520's are beloved and bragged on by all who own them.