I have neither and I have only been geocaching for two weeks, but here is my two cents worth:
If you only do casual caching then the 210 would probably be enough. I have a 100 and it get's me where I want to go. If you do extensive caching or want to save all found caches in the GPSr then the 400 is probably a better fit, due to the extra memory. Both models have a computer connection (my 100 hasn't) so I would think that the computer would be a better place to save all found caches. Standard batteries are probably better than Li-ion when you forget to charge them. It's easier to find standard batteries than a charger when away from home. The extra battery clip is too easy to forget to bring with you.
If I have understood correctly all of the Magellan GPSr of today have the same receiver (GPS chip) so the accuracy would be the same for these two units.
If price matters the 210 would probably be the best choice, if not then you will get more features from the 400. If you need the extra features for caching is uncertain.
/BengtB