I would not go for the Bear, since it doesn't carry a lot. The chance of nuking someone is too small to be measured, so getting the Bear to fly far and drop 2 bombs is just too uneconomic.
While both planes can carry long range ASMs and LACMs, the Backfire carries much more missiles. And since only regional range is needed, getting the Bear can scare the smart ones really bad. Spares for either plane should not be a problem as Russia has a lot of planes sitting around.
One big con for the Backfire though. That thing is a big gas-guzzler.
I don't think China should get the nice looking Fullback. They already have the JH-7A and MKK. The Su-32s have the same capabilities as the MKKs, with the exception of better luxuries. And what will you arm them with? You are just grounding the planes once you run out of Russian ammo.
The JH-7A is underpowered but it's much more reliable in terms of weapons and supplies, and it serves as a better fighter when in need than a Fullback.