To answer your first question, the 780 ti is obviously better, so the real question is, do you want to spend the extra money.
The 780 ti is roughly comparable to two 770's running in sli.
I was debating whether to upgrade to two 770's or just a single 780 ti. I went with the 780ti last night for an upgrade from my dual 570's.
As for you second question... Only you can answer that one. Personally I didnt "need" to upgrade, I just "wanted" to upgrade.
I don't think you "need" to upgrade, so I cant say whether or not its logical
also unless I know what you intend to do with your card... what games you play and how bad do you want to play with all the eye candy turned on in those games.
For me BF4 was running great on my rig with medium/high settings... But I "want" more.