There are virtually no "pros" if this happens.
I did a blog post about the options the SEC has. If you check my post history it's in there right before this post. It spells out some options the SEC has.
Bottom line is the SEC has no real good options here. The SEC won't "kill" Ethereum, but making it a security means the initial sale was illegal. This will open up all the founders to lawsuits by those who lost money at any point. Those lawsuits combined with the fact that legitimate exchanges will have to de-list ETH and ETH pairs until they are licensed to trade securities.
So if ETH is a security, even if the SEC just gives them a fine, the lawsuits would do them in. Anyone who lost money would be entitled to reimbursement.