Opera 12.17 is still a great browser, and I keep using it. But there is a increasing number of sites that don't work well in it (or don't work at all)
Opera 24 is based on a different "platform", and it seems they are working a lot to make it better. But IMHO there's much to do to reach the older version level.
I don't understand why this information isn't provided when you check for updates in 12.17. Seems simple to do (but I'm not a programmer) and the most correct option.
I "discovered" the new version almost by accident, and I'm working with both (in my case the new installation (v 24) kept unaltered the older one (v 12) and that's good, mostly because the new one don't have (yet, some say) a fine bookmark manager, relying on third-party "Extensions")