Having the same number of teams in both leagues makes perfect sense. I also like the idea of having only the best teams in the playoffs, I REALLY dislike it when one division is very weak yet the winner gets in the playoffs over another that is clearly superior.
There should definately be an equal number of teams in each league. Having more teams in the National League actually put the NL at a disadvantage for the All-star game as managers are required to take a representitive from each team. That meant that there were more teams in the NL that could have had no real star and two less better players could not be selected. By having 15 teams, there would be one interleague series every day which would be better than the format they have now.