Yeah, doc over first aid any day of the week, relying on first aid in TC or PVP will get you killed because it can and does critically fail.
Also I agree with seki's assessment of MOS, and would add stonewall and a decent luck are a must have (stonewall for obvious reasons, but also luck, because armor bypass rolls are usually based off luck, your dr and dt will completely go to waste if you dump luck and get crit bursted for example.
If you don't have at least an average luck, MOS, Stonewall, 2x LG, Adren rush and even tougher you aren't really a proper tank.
Most will say 4 or 5 luck is more than enough, but if you think about it most "rolls" against luck are like a d10 that has to roll under your luck score, some of these rolls have -2 to your luck as well. My tank for example has 8 luck, you heard me 8, as even an extreme bypass crit only has a 40% chance to actually bypass my armor. Dump luck and you will be shredded by crits, you might as well not have tough, even tough and ad rush.