When you pick a side you will get a Splatfest shirt that only costs 2000 to scrub instead of 20,000 like normal gear. The build up to Splatfests is a great time to farm for ability chunks.
Just make sure when you are done playing in the Splatfest that you scrub the shirt one more time as the Splatfest Tee is lost once the splatfest ends along with any ability chunks left on it.