After reading that MotoIQ article, I'd get the RS.
As for engagement... Basically what I mean is how much torque is needed to lock the differential. When you're grip driving around a corner, you'd like the outside wheel to be able to turn a little faster than the inside wheel. If the differential is locked, then both wheels turn the same speed and the inside wheel is either turning too fast, or the outside is turning too slow (or both). When this happens, the car doesn't like to turn as well. It would prefer to keep going straight so the tires aren't scrubbing. This causes understeer which lowers the total cornering grip. If you start spinning the rear tires slightly, you can counteract this understeer -- but because the rear tires are already losing grip, the total traction available goes down. Basically you can't corner as fast.
The goal of tuning an LSD for grip driving is to make the differential lock up whenever you apply more torque than you'd use to maintain speed around a corner. Say that it takes ~100rwhp/100rwtq to maintain speed around a 80mph corner at maximum grip. You'd like the differential to lock up whenever you applied 105rwhp/105rwtq.
If the differential is locking up at only 50rwtq, you would be understeering (or oversteering with throttle) and would wind up slowing down. So you can deactivate clutch plates, remove any extra shims, or you could stiffen the negative springs. If the differential is locking up at 150rwtq, you would be losing torque to the inside wheel when you started to apply power coming out of the turn. So you can then reactive clutch plates, add in shims, or soften the negative springs.
Note that "lock up" isn't really an accurate depiction. In reality, the clutch plates are going to be slipping under low torque situations so it isn't acting like an open diff, nor is it acting like a completely locked differential. It'll be somewhere in between. You just want to make it slip under the conditions where you do want the outside wheel to turn faster, and locked when you want both wheels turning at the same speed.
The problem is going to be finding out just where that point is. I imagine it would involve a lot of trial & error, so if you're paying someone else to essentially rebuild the differential as they swap plates/shims/springs each time you make an adjustment, it'll get fairly costly. And if you made a bunch of changes to the car that affected the handling (say you installed massive slicks) -- ideally you'd want to readjust the diff to compensate.
That's one of the reasons torsens are so popular. They don't use clutch plates, they use a combination of gears to essentially send torque to whatever wheel has the most traction (as long as the other wheel has some -- they don't work at all when one wheel is completely unweighted). Many claim that they're better than a properly setup clutch-type diff (but others vehemently disagree). For us, it really doesn't matter since the stock MK3 1JZ torsens are pretty weak and no one else makes one that fits.
