Drop In
+ protects bed better than spray in if something heavy is dropped in the back of your truck as it can flex more and absorb the hit, whereas a spray in will more likely cause a dent.
+ typically costs quite a bit less, the last drop in was less than half the cost of a spray in.
+ because of the ribs sit higher on most drop ins it helps keep you stuff from sitting in water as long if your nose isn't pointing downhill.
+ easier to rinse/clean out because of the ribs.
+ easier on the knees in crawling in the back of a pick up with a canopy.
+ things slide easier. My drop ins I could generally just push most things in. The old ones were really slick, too slick. The new ones are a nice mix and just about right.
- holds water if the nose is pointing downhill. Depending on what's in the back of your truck that can be bad in a downpour, granted one can drill a hole, but most don't. I had some tool boxes that ended up being immersed in water and therefore I ended up drilling holes in every one thereafter. Had a canopy on, but it leaked at a side window and tailgate...
- moves around and will rub thru the paint and create bare spots, you can prevent most if every so often you park with the nose uphill and then lift it and spray water under it until it comes out clean. Did that on my last truck and only had a couple of light rub spots around the wheel wells.
- some accessory mounting stuff really doesn't like the added thickness of the drop in and won't mount.
Spray In
+ minimal long term maintenance
+ looks clean and no rub spots to worry about.
+ spray ins I've had don't allow the water to pool up in the bed if parked with front end pointed downhill.
- cost
- things stick to the bed, so you don't just push/slide things in near as easy. Nice to have things hold, but these are almost too sticky.
- very hard on the knees if crawling in the back.
Bed Rug - haven't had one but a number of people I've talked to like them, but I would suspect they'd still cause rub spots, but not as bad as the drop in. They tend to be almost as much as a spray in, but easy on the knees and guys claim there easy to clean out.