Ships & Modules

That depends,

In PvE content you can find out what type of damage the NPCs will deal and set your resistance profile to be highly resistant to those damage types and you will be able to go farther with less effort. Tanking for damage types you are not taking in normal situations is wasted effort. For example if fighting against Blood Raiders NPCs they will only deal EM/Thermal damage and so you should only be fitting for EM/Thermal resistances. So for this example we'll assume an armor tank, a dedicated armor EM hardner will give substantially more armor EM resistance than you would get as the EM resist portion of an Energize Adaptive Nano Membrane meaning you need far fewer slots to get the same EM resistance from a hardner than you would from omni-resist mods which in turn means your overall fit will be more effective since you haven't wasted fitting slots on resistances you aren't going to use allowing you to fit more damage increasing modules perhaps. Also in this example, your Kinetic and Explosive resists will not be used and so they can safely remain low (low, as in default values).

When fighting players however you never know what you'll be up against and so you do want to even out your resistance profile somewhat. Whether you decide to shield tank or armor tank your tank will naturally be slightly more susceptible to one end of the damage spectrum or the other and you shouldn't worry about that part, the key is to not leave any glaring holes. That being said as a guideline a resistance hole is a resistance that is ~7-8% or more below your next lowest resistance and you should do something to even that out if you have a gap like that. An appropriate shield or armor resistance rig will patch a resistance hole like that without sacrificing much from the rest of your fit and without a great deal of cost. In general Invulnerability fields will be the friend of a shield tanker while Energized Adaptive Nano Membrane will be the natural partner of an armor tank in PvP.

As someone else said earlier there is no ship in this game that does everything well. Everything you add to increase your ship stats in one area decreases possibilities to improve in other areas. This is by design. The key is to decide what roles your ship needs pick a ship that has bonuses that will help you achieve those roles and tailor the fit to that purpose as well. Only experience can teach you how best to approach your specific situation. We can give you some advice but experience is the only real way forward as the range of possible scenarios in EVE is far too broad for us to speculate on your situation specifically. While the early trial and error might be hard it will teach you how to approach not only your current problems but those you encounter later as well. Good luck.