Either way, unless your game has a specific need (needs scrollwheel or key presses, needs multi touch, etc), I would recommend building for all devices. This just makes it easier to work with and you don't need to worry about going back after you've forgotten a majority of the code to make it work for mobile. As for monetization, as stated above, mobile is the easiest but I once planned on having my HTML5 game being monetized, then changed my mind to selling it to a publisher and was happy to have it already mobile + desktop ready! One of the big pluses about HTML5 is the flexibility so don't throw that away!