300Tdi is newer than the 200, but even then it's been out of production for 10 odd years.
IMO the 300 is slightly smoother than the 200, slightly less agricultural.
OVer here (UK) 300's are more easily available just because of the sheer number of rusted out discoveries (the disco 300 is exactly the same as a defender type, bar the engine wiring harness... disco 200's have a totally different inlet/exhaust manifold/turbo and front timing chest, which although can be made to fit into a defender, needs considerable faffing about)
The "joy" of the defender spec 200Tdi, is that if you already drive a 2.5 4cyl diesel/petrol, then the 200 will fit straight into the space previously taken by the old 2.5, and bolt up to the gearbox.
IF you want to fit a 300, then (to fit onto an LT77) you'll need to whip the flywheel housing off of the old 2.5 engine and bolt it to the 300, then fabricate some new chassis mounts to bolt the engine too, as the old 2.5/TD ones are different to the 300.
But, bottom line is, you don't need to change the gearbox to an R380 at the same time (although it is a nicer 'box)
How to fit a 300 to an LT77
How to fit a 200Tdi
Both good reading!