Well if you're interested, here's my take on the whole thing.  I should warn you, this is definitely biased towards the ARB. But we've all heard how (in general) the Detroit is indestructible and the ARB is prone to failure.  There are many many Detroit owners out there that will come to its defense.  So, for the few, the proud, the ARB owners, I submit the following.

Off Road Performance - Ok, before we get to the reliability (not ignoring it, we'll get to that), let's just talk about performance.  There is absolutely no slippage in the ARB and very little in the Detroit.  What little slippage there is in a Detroit is hardly an issue off road since I don't know of anyone who's ever seen a tire spin independently on a Detroit locked axle.  So, as far as off road performance goes, I think we all can agree, both work beautifully.

Street Performance - Performance of an ARB in the street is a non issue (you simply turn it off).  Performance on the street of a Detroit is absolutely atrocious.  Sure, adjust your driving habits, don't put the pedal down in a turn, expect it to pull when you get on the gas cause it'll get squirrely, be careful on ice, snow and rain.  That my friends, is poor street performance.  Sure, it's tolerable on a trail only rig, but I know damn well you guys are proud of those rigs.  I'm sure most of you, no matter where you're going, you'd rather be driving your Jeep.  So I'm not convinced all of you are just driving these Detroits to the trail head.  I've heard some of the horror stories of the Detroit on the street.  One of them that comes to mind was a U-turn with oncoming traffic.  How do you propose accomplishing that maneuver without pedal?  Add a slippery surface.  So here, I'm sure there isn't a lot of dispute, the ARB is superior on the street.

Reliability - Ok, now for the reliability.  First off, the air compressors provided by ARB these days are crap.  Anyone relying on it is absolutely bound for failure.  This excludes the old style compressor with the cast exterior components.  I'm using onboard air, "that" took care of "that."  The air lines are another I've heard are prone to failure.  I haven't seen this yet, but I'll assume it's true.  But, having heard this, I've taken special care to route these air lines right along with the brake lines, which I must assume are in a safe place.  I've also loomed the entire length of the line from diff to solenoid.  Solenoids!  Now these are definitely another problem area.  Carry spares.  Hey, what's the big deal, you don't spare anything?   I've yet to have one fail, but I have a friend who's had two fail.  Now, let's get to the locker itself.  Funny thing is, prior to my little experience, I really was not aware of any high failure rate of ARBs other than the air system.  Now I am aware there has definitely been a problem in the older ARBs with the cross shaft that failed on mine.  It had to do with a tempering problem with a batch of these little babies.  Now I've heard of Detroits failing too.  Not many I'll grant you, but they have been known to fail.  I'd have to say, other than the problem I had, I doubt the rest of the locker components of the ARB are any less indestructible than a Detroit.  This is based solely on examination of the two lockers and not trail failures, I don't know of a lot of ARBs to use as a cross sample.  But still, I've not heard a lot about ARB failures from any of the Internet forums and it's a big place.  I believe most ARB failures, above and beyond the same types of failures a Detroit can have, are in the air system.  I believe the air system can be constructed to reduce problems, but it is an air "system" and is likely to have problems.  I believe most can be fixed on the trail, but they still have to be dealt with.  So for reliability, Detroit wins.

Impact - Now, assuming all the Detroit locker owners were absolutely right, and the Detroit is indestructible, consider this.  The locker is absolutely abusive on drive train components.  We all know it's not recommended you run in four wheel drive in high traction situations because of the detrimental affect it can have on drive train components.  Why would locking your rear axle on the pavement be any different?  I've heard the snap crackle pop from those Detroits.  And those noises are stress relief noises.  Until it is relieved, that stress has to be transferred to other drive train components.  So consider, how much more likely are your u-joints, axles, etc. likely to fail on a Detroit locked axle, than on an ARB axle?  Let's not forget the tire wear issues either.  Deny it if you like, but I know you'd rather not have your axle binding up on the street.  So as far as its effect on other components, the Detroit loses.

Summary - In the end, if your Jeep is a trail only rig, the choice is obvious.  Go Detroit.  The ARB is a lot of work to make a reliable air system.  And it may be possible, putting the air system aside, the ARB locker itself might not be as dependable as a Detroit.  But that is not proven fact.  There's just so many Detroit owners out there ready to beat up an ARB that any failure, air system or not, is just a failure and so the hard components of the ARB are not given due credibility.

If it's a daily driver, the choice could still be a Detroit, if locker reliability is numero uno.  But I insist, no one knows for sure if the Detroit is really more reliable since the implications to the rest of your drive train on the street are unknown.  But if you're looking to keep your Jeep as an "all terrain" vehicle (rock, dirt, mud, sand, street, snow, whatever!), better go with the ARB.

For me, if my YJ ever becomes a trail only rig, and I decide to give up trips to the snowy mountain roads during hunting season, and I can learn to tolerate snap crackle pop and poor handling in my trail only Jeep, then I might go to a Detroit too.  Basically, I see the ARB as a luxury.  And with anything, when you add luxury, you add complication, which adds probability for failure.  But this luxury lets me have peace of mind on all surfaces in my all terrain Jeep.

If it's any consolation, I almost went with Detroit EZ lockers.  I knew they were milder than a Detroit and I felt I couldn't afford ARBs.  I decided not to compromise and waited.

More locker info:
http://www.mswin.net/users/cuetips/jLockers.htm