I say both... I always have the same dilema when making a fit.
without resists your tank will go down much faster.
Without hit points you have less hit points to take.
I've always found keeping the resists above 60% across the board to be a good first gambit, it won't always be possible, but once you get over this I would then definately consentrate on hit points first.
ultimately a lot of the time this decision is made for you due to CPU and Grid restraints.
Shield is different from Armour due to the fact it regenerates, so you tend to stick an IFII or 2 on and go rest extenders to get maximum hp.
If you know what your going up against you have the advantage and can change the omni tank to more focused and get better resists for the same bang.
But ultimately there is a definate trade off, especially with armour, do you go high resists or high HP if you can't go both, then look at the class of your ship!
If your ship is a frigate, it's never going to get much HP, I would always concerntrate on resists, because ultimately if you take a direct alpha blow from a battleship your dead anyway. Most blows will be glancing due to your speed, and better to minimise the impact of these blows then worry about being able to take to many of them.
A big ship that can fit multiple 1600 plates I would fit the extra HP because ultimately again if your primaried your dead, but resists are less cruicial as being able to sit there and absorb the HP for extended periods is nice... especially as the buffer will mean more hope of getting remote rep.