For most of the the time that I have played leagues, there is a major difference between BL amount of people and CL amount of people.
Assuming:
1) Last league's set of participant numbers was an anomaly and it goes back to previous expected BL and CL participation numbers.
2) More people play to get to the 30 for the participation prize.
Then CL will probably be negatively affected in terms of prize distribution, as while it is a random draw, more lots should theoretically be coming from BL than CL. This means that BL will on average most likely receive more prizes than CL.
On the prize given:
There are/was two ways outside of some drawings to get shard codes (I think SG did one war betting that gave 10 shard codes to the winner?). One is through donation. The second, was to rank high in leagues. Changing the ability to get shard codes outside of donating to a random draw is at the least annoying to people who cannot donate and wish to obtain a full set of foil chimeras.
Electrum rewards are a much smaller part of the things you want to win from leagues and make a smaller impact on the prizes on the winners. Not to mention a participation prize being more valuable than actually ranking from 4th-6th in a league.
Outside suggestion: Have a history of previous league rewards. As even though they aren't being used, reference to them is good. I cannot remember all the prizes for the last league or the previous leagues where BL and CL did not have an equal reward system.