Hi Tom,
Good question! Just checked the Play Store stats and the downloads are at around 35% tablets but... a lot of people download the app to their phone initially to play and then end up getting a tablet later on so they can fit more work on the screen (and then switch completely to the tablet but keep the app on their phone just in case), so I think that skews the stats a bit. IMO you're best off running it on a 7" tablet = best compromise between phone and tablet, you can fit much more work on the screen than on a phone but still fit it in your pouch or a large pocket.
Used to recommend the refurbished Hudls on the Tesco eBay outlet store as the best balance between cost/quality (and lots of people using them with good results) but seems Tesco aren't selling them anymore, presumably as they're pushing the new Hudl 2
Samsung Galaxy Tab 3 seems to be a good bet though if you can pick one up cheap. Or go premium and run it on an shiny new iPad, all down to personal preference/budget really.
Rob