Yes, that's what I meant above with "common sense".
When the modifier % are the same for both stats (for example Dragon Kick, it's 50% both STR and VIT) and stats on items for that slots are very close, normally STR is preferred because of fSTR and because STR naturally gives attack. Say for example a STR+5 helm and a VIT+6 helm, I'd probably prefer the first. Now put a STR+7 and a VIT+15 helm in comparison, I'd probably go for the latter.
But this isn't what I wanted to know. It would be hard for me to explain even in my mother language, it's definitely harder for me to do it in english.
Let's try with a graphic (numbers are completely made up, it's just for the sake of making an example)
http://i55.tinypic.com/1jnuw8.png
Now I imagined there is a "threshold", which is the red line. This threshold is somehow related to the sum of both stats. The higher BOTH are, the higher the threshold will be. If one stats gets too far away of the other, the threshold will be lower.
Adding too much of a stat over that threshold makes you get diminished returns from that stat, or even no return at all, as if you didn't add anything after a certain number.
Now if you could somehow raise the other stat as well, in this example VIT, the threshold would go up as well, and you would get a higher return from the STR values, which were in surplus before you raised the threshold.
I have no clue if such a mechanism (which is present in other games, that's why I'm asking) is present in FFXI too. I honestly never bothered/wondered before now, I never found myself not even remotely close to such a situation, until Abyssea and its +120 STR bonus.
Decimals after the first one, I thought? Or at least that's the case for many other stats in FFXI, like TP for example