How much consideration do you give a single IV vs overall IV?

I'd take the 14/14/14 over almost anything else. Really only 15/15/15, 15/14/15, and 15/15/14 are better. IV variations with 14 Attack and 15 or 14 in the other slots are equivalent in my mind.

There is some consideration to be made with short term vs. long term benefits, but if I am going to invest Rare candy into something, like a legendary, I always think long term.

To me, one attack IV equals one overall IV. Ie. 91% 15atk is worth the same as 93% 14atk. Generally I will prefer the higher IV total as long as it has at least 13 attack.

It depends... How soon are you planning on maxing it at 40.
Vs are you leaving it at 30.

I tend to overall if going to 40. And Attack if stopping at 30.

Though again it matters a lot in measure of degrees. Sometimes I just go highest CP which will tend to make
15 Attack 13/13 > 14/14/14
and
15/13/12 = 14/14/14 roughly speaking

I prefer the 15 attack as long as the other stats are above 10 since I don't power up past 31. The best defense is a good offense, as they say.