I think the transfer box change is more reliable, as its fit and forget. The overdrive will require initial fettling on setup and has the potential to be 'another' piece of kit that could go wrong, although I think these modern ones are pretty bullet proof, @Brown has fitted one to his Landy.I actually preferred the overdrive route in my mind( best of all worlds) but our local chap recommended the 1.2 for my use and for reliability. (With remap).
The overdrive is massively more expensive than a recon transfer box.
For ages I toyed with the idea of both/either, but really enjoy the almost 'sporty' feel of my mildly tuned (remap, full size intercooler) 90, even with a trailer, and I do tow up to capacity fairly often, I loose the sporty feel but pulling up hills and starting off is perfect (I guess why they designed it as such) with the 1.4 ratio. I now don't even think about it and wouldn't change it for anything, I also like that I have all original genuine numbers matching.
I would suggest having a drive in one that has been converted already before taking the plunge, you might be surprised how much omph it takes off the Landy! Best wished with it whatever you choose to do, moding Landies to our personal preferences is what it's all about!