First question is do you fit in a Defender and how do you like the ergonomics? I did fit (I'm about 5'7") and thought I liked the ergonomics once a 15" steering wheel as fitted but I discovered after three years of ownership of a 2013 Puma 110 I was carrying an injury in the left leg/hip from the awkward clutch position/weight, and it started showing up as significant pain whenever I drove for a few hours in any car, not just the defender or a manual. So start with the ergonomics.
Which is more desirable depends on what job you want the car to do. a 90 makes a better trail car if your trails are tight, but if you have steep climbs that 93" wheelbase causes lots of weight transfer. It's a pain to get in the back and lots of 80's 90's have a bulkhead behind the front seats that don't permit much seat travel/recline. The 110 is fairly bulky but stronger in the chassis (and rear diff as has been pointed out) They also ride better due to the longer wheelbase, have better fuel range and obviously more storage.
The issue I can see with a 90's or 90's D90 or 110 is it's going to be an import, I assume from europe/UK and therefore it's going to be rusty. The closed box chassis is stiff and well designed but it rusts like crazy. Rear crossmember (well, the whole chassis aft of the rear axle, really) rusts out readily and requires replacement. The bulkheads rust like crazy. They all leak water so it's hard to prevent. We don't have rust issues in Australia, basically, and holed chassis and rusted out bulkheads are still common. My 2013 was starting to corrode the bulkhead around the windscreen "hinge" brackets by about 2 years of age.
You have to be a special sort of enthusiast owner to keep one running well. I didn't bond with my defender anything like I expected, and a couple of years after I sold it, I was able to buy an early style S1 Discovery 3 door 200TDi. It's mechanically the same as a 90 of the same era but is quiet, doesn't leak, has A/C, is miles more practical than a 90, and is pretty much the most comfortable car I own. Yes it takes constant maintenance, but at least I really enjoy driving it, whereas the D110 was a chore.
Personally, I wouldn't buy any land rover as a trail car, they're too weak and expensive to upgrade, but with the prices they now fetch, if you have the budget most problems can be worked around without overcapitalising on the car. At least the 80's and early 90's cars still had the Sailsbury, oil bath wheel bearings, deep drive flanges etc so they're better to maintain than the later cars. Whether you can find one that isn't a rusty basket case though is a different matter.
