Apparently the R380 gearbox in a 300 is a little stronger than the LT77s in a 200 - but then either will break if it's abused. Whether 'stronger' means harder to break or that it will wear out more slowly I've never found out.
300 has slightly more electrickery under the bonnet than the 200, so in theory a 200 should be better for deep wading than a 300. Mind you, with that said, a mate has a 300 which he uses regularly for wading and has never had a problem with electrics - so the electrics thing is fine as a theory but doesn't make a real world difference.
Price is the biggest difference - all the 300's are post '93, all the 200's are pre '93 - cross over was in about March/April/May time in '93. Expect to pay more for an '97 R plate 300 than a '92 J plate 200 for obvious reasons.
Cheers,