PLA has a lower melting point, something to consider if you plan on using your pieces close to your lights (if your lights get hot the PLA will be more likely to deform). Also if you are doing dual extrusion I would not use PVA, because PVA is water soluble. (I assume you already knew that but hey, better safe than sorry)
Some details on PLA vs ABS
glass transition temp: 65C 105C
Slumping Temperature: 75C 115C (this is the important one)
Melting Temp: 160-190C 210-240C (printing temp is usually higher than this so the material is less viscous)
Other things ABS is more flexible than PLA which means it is harder to break but also well.... bends under weight. ABS also sticks to itself better than PLA does (better layer bonding). But PLA is typically easier to print, the colder temps and less flexibility also make it expand/contract less due to heat which reduces warping. Overall, I would recommend ABS if you have experience with it and are comfortable, both will degrade, PLA will probably degrade faster seeing as it is actually biodegradable(made from corn starch), though both absorb water over time and will be damaged by UV light. Also for me heat would be the major thing to consider, but if you aren't using this by your heater (as a clamp for your heater) or by your lights, then it probably won't matter which material you use.