A HTML 5 client is a must in my honest opinion because it is a universal platform and most people would like to play on the go on their mobile browsers and not lug around a specific tablet with iOS or Android or Windows.
+1. HTML5 should be considered as default choice with Android client as immediate second due to the huge number of Android-based mobile device (Tablets, smartphones, etc.) deployed Worldwide.
Full disclosure: I own almost only Android devices.