Hello and welcome ..
I think there is a big preference element here - and some of your preference will be driven by things we don’t know - like how you are going to use it etc …
So I think I would probably suggest two things:
The first is, if you can, drive a few of both of them. The 300 is older - it’s noisier and slower - but it’s simpler and more basic too. My own experience is that the 300’s were a bit better made.. but the later Td5’s had nicer dashboards and instruments etc. There are more things we could compare and contrast - but I do think they are not just binary / better. It’s more about what do you like and what do you want from the vehicles - so try and drive some.
Personally … I love the Td5.
The second thing I’d suggest is that condition is much more important than if it’s a 300 or Td5. A really good 300 will be better in almost every way than a dog rough, mistreated, Td5 …. and thus make a better purchase.
A car that’s been looked after, is honest, straight, good condition, rot free, and mechanically sound is the primary basis for your choice.
So I would suggest you have a clear budget - and just buy the best Defender you can.