I think the difference is the implied likelihood and your personal stance toward "Es".
Es wäre cool is more likely to happen than Es würde cool sein
The "stance" can be explained with a example
When you were just told that the other person will do something for you (maybe something that puts a burden on them) then you can choose:
Es wäre cool: polite way of accepting such an offer
Es würde cool sein: even more polite. OR: implies that you are not comfortable with the dimension (you have the feeling that you are asking for too much)
In the end it kind of depends on the way you say it and the context it is said in.