FIBERGLASS but thats just my opinion.
Flares are nice but Fiberglass looks cleaner.
The cheapo glass may not line up perfect.
I used AUtoFAB glass on my BII and it was expensive!
The drivers side didnt line up well at all at first, but it wasnt the fiberglass' fault, it was my truck.
I got in a monor wreck a few years back and the drivers side door got tweaked. I re0hung the door (PITA) and now it lines up great!
Flares are cheaper, but you will have to cut the metal to fit them, plus I dont know of many flares made for Explorer's???
If you do go flares then at least when you cut the metal do it right, clean up the new bare edge and paint it, to avoid rust. If you bolt the flares on, same thing, dont leave bare holes in teh metal or they will rust, treat them first.........
Fiberglass has advantages, no rust, they are durable, and they look great.
Flares have advantages, they are flexible, cheaper, and provide coverage/clearance.
Matter of opinion really, both are functional.
I know berfore I did my fenders my tires stuck out about 4" past the feders on both sides, that really killed my paint job and would be illegal in many states. The fiberglass solved that problem............