The LT230 lets you manually lock the center differential (CDL aka Center Differential Locking) in both Hi and Lo. The BW has a viscous coupling (VC) which that you cannot control when it engages.
Suppose you need to lock your center diff and you have a VC TC. You are at the whim of the mechanism. With the LT230, you can lock (or unlock) you TC whenever you want.
The LT 230 is know as being pretty bullet proof. The 1987 and 1988 Range Rover has the LT230, while the rest have the BW. The Disco I has the LT230, which is where most rangie guys get their LT230 for the swap. Some of the DII's have the LT230 but it is not hooked up. I think the late 2001 to 2003 DII's do not have the LT230. In 2004, Land Rover put the LT230 (and hooked it up too!) back into the Disco II.
I think I have my years correct. I am sure someone will correct me if I'm wrong.
The Borg Warner unit was introduced on late RRCs and continued to be used on the P38.
The viscous coupling in the BW transfer box is permanently engaged, giving full time 4wd. It merely acts as a limited slip diff. The BW unit is gear driven, making it quieter and cooler running than the LT230. The BW unit also tends not to give any problems with main shaft spline wear on the gear box, which is a well known LT230 issue.
LT230s continued to be used on Discoverys (1 and II) and on the current Defender because of their price to the factory and because of the cost of maintenance (a new non-genuine viscous unit for the BW transfer box costs over £300 in the UK, never mind abroad).
The LT230 has a couple of advantages in being available in lower ranges for Defenders (1.4:1 and 1.6:1), and in being able to accept GKN or Rover drive overdrives in 1.4 format. It also has the relatively minor feature of allowing safety inspection brake tests to be conducted on a single pair of wheel rollers with the diff open, while such testing could damage the viscous coupling of the BW unit (requiring a Tapley test), and the fact that a cautious drive home with the diff lock engaged after shearing a half shaft is not going to harm the transfer box, while doing the same with a BW unit could damage the viscous coupling because of the continuous shear force.
BW units don't get retro-fitted to Defenders because of their price and because they would leave the final gearing far too high. Few people fit them to Discoverys because most discovery owners are not enthusiasts and have no idea about the BW unit or the fact that it could be transplanted. I have only ever heard of people retro fitting LT230s to late RRCs if they have a faulty BW unit and have cheap access to an LT230 and the car isn't cherished.
The rest of the BW unit is very robust, and while the chains can eventually stretch a little, I still have very little backlash in my RRC after 170,000 miles, with the original diffs and transmission still working well. I can't imagine an R380 or LT77 main shaft surviving 170,000 miles on an LT230.
For what it's worth, the Discovery II had the standard "Q gear" (quiet gears, with finer teeth) and a 1.22:1 ratio similar to the earlier RRCs and the Discovery 1. The first year or so had the selector linkage to the diff lock omitted because LR thought that the electronic traction control would eliminate the need for the locking diff, but they soon found otherwise and refit the selector linkages on subsequent years. It wasn't uncommon for UK owners who used their early Discovery IIs off road to retro fit the linkages to their otherwise fully equipped LT230s.