Imo the PLD af is the worst of all the AF. A good melded set is much more beneficial. Ultimately that goes for all of the classes, but it's especially true for PLD. As said above, if you're gonna keep one piece, make if the head piece. For feet, there really is nothing else better than double melded sentinel's feet aside from darklight. Don't buy them, they are too damn expensive. But if you have a few sets of gear to spiritbond, I'd recommend taking that gear out with a group to the sentinal's sabaton camp and hanging out there for a few hours until you get a few pairs to try to meld. If you're the main tank for your LS, ask them to bring you out there and donate the drops to the cause. Ultimately, it will benefit them. Even with a single meld, it's hard to argue against the usefulness of the extra 9 enmity. For hands, go with whatever. I've got a set of double melded sentinel's gauntlets, but they don't really offer much over the cobalt ones with the same meld. At the end of the day +5 block is negligible. You can easily use cobalt guantlets because they are relatively cheap to make so getting a double or triple meld on them should not be too expensive in the end. As for the body, I use a double melded cobalt cuirass, but the pld body will get you through content until you can get one of those.

I created a list, which you can see here, of what I think is the best in slot materia for PLD. I took cost into account somewhat when ranking materia as well.