For multiplatform games you are still better off with Unity imo. Sure, from a developer standpoint, building on open tech is cool, but gamers don't care at all and when Unity gives you the best possible performance and development workflow i would stick with it.
They will also probably soon have a HTML5 export (they already have PNaCL)
For performance you're better of with Unity. For distribution you're better off with HTML5. You can get in the app stores & have distribution on the mobile web, have the game instantly playable in webviews, etc...
Sounds great in theory, in practice mobile web views are pretty slow for games, as op already stated. Besides Unity will have a HTML5 export soon which will make it the most efficient tool for creating web games anyway.
