Ships & Modules

Armor plate simply increases your base armor amount. So 800mm plate will provide less buffer HP but you'll also end up moving quicker because there's less mass. In general the choice of going buffer tank or active tank depends on the ship and the intended use. If you give more information about what you're trying to do, people on here will be able to provide you more accurate suggestions about what you should be fitting.

you repair at the exact same rate regardless of the plate size, the larger plate just gives you more HP.

typically your not going to combine a plate with a repper on anything larger than a frigate (plate + repper fits are popular on solo rifters).

repper fits without a plate have the advantage of no speed penalty (assuming a proper repper fit with nanobot rigs), they can be quite viable for solo or small gang work especially on bonused ships like the myrmidon (with cap booster and dual or even triple reps), the lack of buffer tends to be a problem in fleets where your likely getting primaried by the entire opposing fleet instead of being shot at by 1-2 ships. Armor buffer of course kills your speed.

In general fit the biggest plate that will work with the rest of your fit, you may elect to go with a smaller plate if you want more powergrid to fit larger guns on a gankier fit, and a larger plate or even multiple plates (in BCs and BSes) if your going for a bait fit or some type of super heavy tackler where the dps loss of smaller guns is acceptable.