You could always just try it out on one planet to a certain extent, then delete your character and create a new on starting on one of the other worlds.
I’d say they gave only 2, partly so that the landscape on one planet was not completely flooded with just a few players main characters and their alts, but also in part, that you could potentially (although I haven’t checked!) have every possible skill path with an additional alt on the roster.
It’s definitely not done due to zone count, as there were many more worlds/zones earlier on in the development, and I would imagine that there will probably be many more localised zones upon release as well (I’ve seen the question asked somewhere regarding which other real-world server locations would benefit the community).
That being said, the intention for 1.0, is that all starting worlds with be equal in resources distribution, landscape, creatures and general feel … basically “earth-like”, which they mostly are at the moment - with the exception of the sky colour and grass variation - so there’s not really much else that’s different.