Hi Mick,
I'm in the "fortunate" position of owning both a 101 and lightweight.
The lightweight is going to be a better bet for everyday use. More commonality with series 3 land rover parts, and hence cheaper to fix.
Yes a lightweight would also be cheaper to run as regards fuel unless you go to LPG.
If you go for a lightweight make sure the top bulkhead vent panel is in good nick. This is the hardest part to repair, and expensive to replace now. Rest of bulkhead is straight forward flat panels. Chassis repair parts are mostly common with other land rovers.
Chassis repair parts for a 101 are more difficult. I waited 7 months for a rear crossmember from the 101 owners club!
One of the main areas to watch on a 101 is front swivels. these are part of the axle, unlike most other land rovers.
Good luck with your land rover purchase and enjoy one of the cheapest and most practical military vehicles available.
Norman