I have a Nexus (very similar to the new Tesco tablet) and have downloaded the 11+ Apps from this site. I have to say they are brilliant! My DS much prefers doing prep work on the tablet as it's somehow much more fun than doing it in a workbook. So far each "test" is taking about 30 minutes which is perfect. With the vocab builder app I've been doing these tests with him, so we can discuss each word in the answers to check whether he knows what they mean and look them up in a dictionary if need be. So I'm hoping this will really help him to expand his vocab.
I don't have an iPad so couldn't tell you how this compares but when downloading the 11+ Apps from this website, I also found lots more out there, so I wouldn't say you'd would be restricted by going android.
Hope that helps!