My main problem was this: A) I had to fight the Eleutheroi army near Chersenessos too (it attacked me), so didn't have enough money/men, to actually go take Sinope, It got a pretty large garrison.
b) I'll need a cavalry wing later in the game, and Skythian nobles would be perfect for that, Olbia would make a great place to build up, as a place to recruit those guys.
c) the seleucids took my capital right after I got chersenessos, so If i took sinope, I'd be in war with them much sooner than i wanted to, and the Sauros would attack eventually.
Hence my choice fell on the weakly garrisoned, but rather large, city of Olbia :) Your game might be evolving differently, just throwing it out there :)