Hi, rjr - I believe either one will do. Technically, you can't go wrong using Custom/Mod BST. But even though it's a mod, I'm tempted in this case to say use WTS at the MP as a basic ROP isn't too tricked out, and may reach a larger audience of those looking for stock looking, and/or partially modded lights. Also, it's not uncommon to see ROP's, 1185's, 623's, and whatnot being sold there.
You should be fine either way. :)