I went with ovrlnd campers myself, had looked at a few of the others but ovrlnd had everything I was looking for, and lots of custom options.
Congrats on the order! I have an ovrlnd too and imagine you are gonna love it. Currently building out the inside. Have it on a f150 6.5' bed and absolutely love how much damn room we have. Previously had a gfc on a long bed taco but got tired of feeling so cramped and the terribly uncomfortable seats in the taco. From experience with both, i really like being able to have the full bed open to use when the platform is slid forward. Also love being able to have a 3" mattress and still keep bedding up top. The whole point in my mind is to make camp setup super easy and quick so having to move bedding every time was annoying to me. Also not moving the cushion to get up in the middle of the night and piss is great or to start coffee in the morning.
I think personally the ovrlnd is better than project M as well just because it is truly custom fit to your truck so maximizes the space. also not sure if you can get side hatches on both sides with the project M which was a must in my opinion.
BTW i am in no way affiliated or sponsored by ovrlnd, just a happy customer. Jay is great to work with and has continued to be responsive with all my questions regarding the build out and is just a psyched dude.
Only complaint thus far is dust. this is likely more related to the F150 tailgate which is ridiculously hard to get sealed up rather than the camper and i imagine any of the similar products would have similar problems. I am actually about to go crazy and replace my half barn doors with a full vertical hatch (or possibly full barn doors) so i can get rid of the tailgate and build an aluminum frame to seal against around the tailgate.
in the end, each of the campers have their advantages. If you want a simple, super light camper that is lower profile, gfc may be better. if you want to maximize space of the camper and care about leaving bedding setup, go for the ovrlnd. Just my 2c! happy to answer any other questions as well.