Even though everyone tries to sell ZK with its mechanics and UI, I went from BA to ZK just because of the nice graphics. To be more precise: after I've seen the model and firing effect of the Leveler.
For a complete noob both of these games are the same, while zk just has superior graphics. BAR might change this though.
Now that I'm used to ZK, I can't enjoy playing any other RTS "competitively". The amount of power a single button press in Spring's UI has can just not be matched by any other RTS that I've played. This means that even when the game is slow paced and you have more than enough times to micro your units, it is just plain annoying. Could you imagine, that there are games where you have to manually click on each low hp-fighter in order to retreat&repair it :O
Then there's the community: One can have a lulzy evening with the pluks, relax in a coop, do ai battles or rek sum nubz in (small) teams. The player count is just big enough to allow for such a variety while still being so small that everyone knows everyone, which somehow gives a feeling of being at home :) Most other games have either hardly any or far too many players. I think it's also special that there are no serious trolls/griefers due to the strict rules. You can imagine how hostile some players get in other competitive games if they have the feeling that their teammates are underperforming (wub wub, knob, .. is nothing compared to that). Additionally, it seems like the zk community has a common denominator when it comes to programming.
