I have an explorist 210, which seems to me, looking purely at the specs, that the 210 has a lot larger memory area (22MB compared to 8MB for the 300)
The extra bit that the 300 seems to have is a barometer and a thermometer, but appears to have no bigger screen. BUT it can tell which way you are facing when you are not moving, the 210 can't do that (but walk a metre or two, and will soon tell you)
From what I can see, the 300 was meant to be an improvement over the 200, but from what I see the 210 is a better unit, unless they decided to release a 310 with extra memory like the 210.
Consider this also, they don't have a Euro 300 model, which may suggest that the 210 "superseded" the the 300, but has a slightly lower quality GPSr chip