I agree with @neduejianya. I wonder what could possibly be the reason @acidyo for the dropped frames. It could be frustrating. @neduejianya. Well there's two choices for you, Tank Plate (Burnished X of Might) or DPS Plate (Polished Y of Valor).

Now just skimming over their toolips the differences between them seem to be negligible now that stats like dodge and parry are removed. One is haste + some crit, one is all crit. Maybe the stat scaling on them is different though, I don't know. I thiiink haste is better for prot pally but I also don't think it's a big deal at a low level. I remember tanking through dungeons in Intellect Plate for a while. It was fine until about Lich King content, but around then you should be replacing a couple of heirlooms anyway..* This was also before the stat squish so you might not want to try that.

It looks like heirlooms now cap out (stop scaling with you) at 60, unless you upgrade them. If possible at least put them to 90

I could go through the rest but there is only 1 heirloom set for each role. Also I think strength DPS set is more versatile because its stats help both tanking and dps, whereas bonus armor heirlooms don't do much for a ret pally. So it's really up to you.

If you can only afford a few pieces, weapons don't give you an XP boost and misc gear like rings/neck/trinkets are just gravy. Chest/pants/helm are the most powerful, with shoulders behind a little.