I have tested most of them for work and to start with don’t worry about the screen quality. They are all good enough and you really don’t notice the difference. Its just a talking point for geeks. A bit like megapixels with digital cameras.
You wont have any compatibility problems with iPads or any of the devices.
Microsoft surface is a great idea but it runs slowly, takes 5 seconds every time you want to open an app which doesn’t sound much but gets really annoying. The keyboard however is amazing.
Kindle and Nexus are great but seem quite basic and there isn’t the range of apps for them. This might not be a problem for you but worth checking what portfolio apps there are for showing off pictures.
Personally if you are using it for business I would go with a iPad. iPad mini is great but g for an iPad 2 if showing off pictures. Hardly any difference between the iPad 2/3 and now 4.
Best idea is to order all of them over the internet, give them all a try and then send back for a refund the ones you don’t get on with. Remember with mail order you have 7 days to return an order. Most companies however give you 14days to notify them.