I've just got back from the Microsoft Showcase Classroom Roadshow, and to be honest I was blown away by what is possible on a £150 windows tablet. However, I don't wish to say one tablet is better than the other (and I, like many other teachers, have pretty strong opinions on the the matter), rather I want to suggest a few rules that might help in the decision making process.
1. Choosing tablets is rarely a department only thing, usually a whole school rolls out tablets as part of a cohesive IT kit strategy
2. Work out how you are going to use them - offline/online, one device per child/class sets, content creation/content consumption etc
3. Check you network infrastructure, particularly wifi, can handle the new devices
4. Decide how you want the tablets to interact with other digital devices in the classroom, this is probably the biggest deciding factor in OS choice
5. The golden rule- your tablets must be able to flex to accommodate changes in the future - you are going to be spending a lot of money and it better last. You don't want to discover the tablets are going to be no good 6 months down the line. If the technology doesn't help pupils learn, is there a point to using it?