Best mobile device 4 I/R @ trackside pls

I'm looking for a mobile device that I can use to' lay in running' whilst at the tracks. Phone or notebook etc, I don't care just so long as I can get my bets on when I need to.. i.e when a horse I am aiming to lay, pecks on landing and I react by pushing the submit button. Anyway I'm sure you get my drift but I think I would prefer a phone for convenience sake.