Crosswalk signs it auto for you, based on your account so you can update your crosswalk compiled games any time. But its a different key than to your own that you sign the CJS apk with so you cannot switch on Google Play.
As for the two.
CJS = Great for smaller games, its got Ad and Google Store support. Cannot do large games due to its lack of proper memory management. It will crash even 1GB devices. Also has 7 required permissions by default which could put off some users.
Crosswalk = Great for all kinds of games BUT currently does not have proper Ad or Google Store support. It does have leadbolt banners working, but I rather use AdMob.
Currently, small games with monetization = CJS. Big games as a paid app = Crosswalk.