Contract Balancing

Is there a use to balance out contracts between the different size of planes? For example, what percentage of contracts at INN should be S and M? While I certainly wish to raise my XP at the airports, I am more interested in silver planes to get to gold. Do larger planes give more silver planes than smaller?

Here’s the screenshot of the difference between S and M:

In my opinion S is best for XP and M is better for SP