Depends what you want it for really.
A defender would be nicer to live with every day as a commuter, more comfy, better ride with coil suspension, holds the road much better than a series, especially at higher cruising speeds.
Also comparing both as bog standard with no mods, the defender is better off road, as it had greater axle articulation, and no leafsprings under the axle to get in the way.
Having said that series have more charm, and a more oldy world agricultural feel. Most people will tell you sereis are way easier to work on, true they are easier, buy not that significantly easier. The defenders have more modern engines which does make them a little more tricky for the home mechanic.
If you want to tow, a standard defender would be better than a standard series, they have much better breaks, and have a bigger towing capacity rating. Series can still tow alot, they just do it very slowly.
If you just want a play thing a series is a whole lot of fun, and can do 90% of what a defender can do, but with way more charm, which will put a smile on your face.
I think if I lost my series landy, as much as I love her, I would maybe contemplate upgrading to a defender.
tell us what you would like the landy for and we can give you our thoughts.