Until recently, the 401 was only available with an auto battery. The 510 was available with a manual battery, which gave you much more control then the 401 auto batteries. So the 510 got a good rep as being a workhorse of a PV, with regard to throat hit, and the 401 auto's couldn't quite match it, performance wise. So the 510 got recommended to almost every new member that asked the question, "what should I get?". And as more people got them, more people would recommend them.
However, now that the 401 has a manual battery option, it performs on par with a 510, with regard to vapor, but still has better flavor, better battery life, is smaller, and also gives a good throat hit. (New manual 401 kits are only available in black, but it's a very sleek looking unit).
As far as switching batteries changing the taste on the same atty, it could be a few things. A new battery replacing an almost dead one could effect the taste and vapor. Or a cart that just happens to be running low at the same time you change batteries. Or possibly a battery that got some juice inside of it could effect the taste. Can't really think of anything else, except that one battery might operate at a higher nominal voltage, or one is dying and runs weak.
