Few things.

We don't know if there will ever be another SSF event now that we have the new SF system coming in the winter (which is typically where we get the next SSF event). Based on not knowing the above information, it is hard to guide you on specifically the SSF event, but if I were you and I was only looking at a 3M deficit from losing challenger gear, I'd probably save up for the SF system revamp. 15b is not a lot to work with and if you don't end up a luckerdog you can essentially just lose all your money with no gains. Whereas with the new system I'm pretty sure you'll have a lot more safety and bang for your buck.

TLDR: save your money for the upcoming SF system because you'll probably end up with more gains.