Armour tanking has it’s own advantages and disadvantages, shield tanking has different advantages and disadvantages. Shields aren’t always superior to armour and armour isn’t always superior to shields. Armour repping can be more efficient than shield repping in terms of how many points of tank you repair compared to how much cap you use to do that repair. Having high damage resistance definitely helps make repping more efficient, but that’s true for both shields and armour. Look into rigs that make armour reppers more efficient, such as Auxiliary Nano Pump rigs. There are also some ships which have built in bonuses to increase the efficiency of armour repper, like the Gallente Myrmidon battlecruiser.
One general rule for tanking which applies to both shields and armour is, you don’t need to keep your tank at 100% all the time, you just need to avoid it going to 0%.
From this perspective, on an armour tanked ship the shield will act as a buffer, preventing your armour from taking damage until your shield is very low. You don’t need to start running an armour repper until the armour is taking damage. If you start your armour repper while your shield is still healthy, you’re just burning up your cap for no benefit. Wait until just before your shield gets to 0% before you start using cap on your armour repper.
My general rule of thumb for an armour tanked PVE battleship is that I fit for around 3 minutes of Cap, meaning I can only run everything on the ship at the same time for 3 minutes before I run out of cap, but with skills trained to a high level and understanding how to manage my tank, I rarely go close to running out of cap, because I can judge when I need to rep and when I don’t need to. As long as I don’t let my tank get to 0%, that’s all I need. Trying to keep my tank repped to a high level all the time just burns up cap for no benefit.
Depending on exactly what I’m doing and how much incoming DPS I have to deal with, I might not even start repping my armour until it’s down to 40% or so for less intense situations, but for higher DPS situations I’ll start repping sooner, around 60% or so.
The point I’m trying to get across here is: It’s not just a matter of having a good tank fit, it’s also a matter of knowing how to manage that fit, when I need to rep, when I don’t need to rep, etc.
Even with an active rep armour fit, there’s more than one way to do it. Depending on the size of the ship (destroyer, cruiser, battlecruiser, etc.) and how many slots you have to work with. you could go for one big armour repper or two smaller reppers. The idea behind using multiple smaller reppers is that each repper uses less cap, Keep one repper running all the time and only use the second repper when the incoming DPS is too much for one repper to handle. Even then, you don’t need to try to keep your tank at 100%, you just need to keep it above 0%.