If you dont have other options available to get Defender, then Nemea would be the more balanced choice. If you can put in a Defender L, then go for Ascalon.
Crit you always have to ask yourself: Do I use my main team for everything or only against advantage? Crit only works against advantage element, in your case Wind. So it can fast lose value if you use Fire against other elements aswell. It is more a personal preference choice to consider.

What exactly do you mean with burst gains? You mean BG generation or Exceed? Goal overall is always to get as much Exceed as possible and needed (main goal is only to reach 100% cap increase, something around 70% is most realistic to aim first at) when you reach the point where you do a lot of damage so you can increase your Burst damage aswell. In an optimal world this is in collaboration with a team that can burst on a regular frequent basis to maximize it's value to the fullest.