I have Detroits front/rear in my Blazer and had one in the back of my pickup/tow rig, if I got another I'd buy a Grizzly.
Not to say that I've had issues with my Detroits because I haven't, but I feel the Grizzly is better about not breaking if/when you break a shaft, it's cheaper and has a better warranty.
If you have a gov-lok, you either need to buy an ARB/Yukon posi (because they replace the whole carrier) or find an open carrier and they are WAY expensive new. The Detroit/Grizzly ONLY fit in an open carrier.
As for the mechanical vs. selectable debate, in my opinion it comes down to use and the person. I put a Detroit in the back of my Blazer in ~2002 and have had one there ever since, I have no complaints with that at all even in tons of snowy Colorado winters (it's different but not hard to get used to).
I ended up putting a Detroit in the front as well (actually tried to buy an Eaton E-locker ~2005 but they couldn't actually produce a 35 spline D60 locker at the time, even with the right application/part number

). That one I've never been real excited about. I took that on one trip and knew that I needed hydro assist after about 2 minutes. So help from hydraulics fixed that and it's been fine for a really long time, no issues, but being that it's locked pretty much all the time makes my turning radius suck sometimes.
However, my tow rig/84 K30 was a CUCV truck and had a Detroit in the back and I hated it, it got the boot. I'm really not sure why this one seemed so different, but it would hop/buck the truck even in neutral in parking lots and it pushed the truck around a lot when towing. It could be the different weight bias between a long bed and a Blazer (both are manual transmissions) but I loved the rear Detroit in my Blazer and hated the one in the K30. Or maybe I'm just getting old

. It's an ARB now and I love it, other than cost, in my opinion that is the absolute best option and I hope to put one in the front of my Blazer next year.