What are the pros and cons of each?
There is no real answer to this, not without first asking and answering some other questions:
1. For use at what off road?
2. Standard or modified, if the latter, how radical?
In short the Defender has more power, power steering, better steering lock, stronger axles and gearbox, more wheel travel, better flex and more suspension travel.
So in general a Defender will be more capable.
However, something like an 80" Series 1 has huge advantages on some terrain. It's a lot smaller in all measurements, so has a better breakover angle and is far more nimble. For trialling or use in tight confined terrain (forrest/woods) it's size is a real advantage.
It's also lighter, so arguably better on lose surfaces like sand.
Downsides are Series 1's aren't very powerful and the standard suspension has little travel, even compared to a Series II (front suspension).
If you allow mods, then a Series can be highly capable and lots of available options to improve them.
But it comes down to what you want to do with them. If you are rock crawling Moab style, then a 100" wheelbase Defender is likely a better bet than an 80" Series. The longer wheelbase will offer greater stability.