They are two fundamentally different ways of wrapping html5-games to "apps".
Crosswalk is basically a full browser engine wrapped into your app, upon which your html5-game then runs.. the advantage is, that it's developed quite far by now - the main disadvantage being, that the app size is quite big, since it squeezes the entire browser into the package.
Phonegap however is a framework that uses a technology called webview - it uses browser technology already present on the phones to display a html5 page or game like it was an app. It's main problem is that it only recently became really useful.
So your choice has a few implications. Phonegap only makes sense if you target the very latest phones with android 5 or larger. For anything older than that Crosswalk is the way to go.