Step 1: Operating system: iOS (very conservative and uncustomizable) vs. Android (catching up with iOS and in some regards exceeds capabilities of iOS... will only get better with Android 4.0)... if you chose iOS here, then Apple it is, otherwise it is between Galaxy Tab and Transformer (Steps A through C)
Step A: If cost is very important than Asus Transformer, it's the cheapest.
Step B: Quality of hardware and design Samsung's Galaxy Tab (essentially similar to Apple's iPad)
Step C: Your needs - Samsung's display is supposedly brighter; Transformer will be one of the first tablets to receive Android 4.0 upgrade; microSD card needs (in reality very little for that, period); accessories available for each of the tablets; and finally brand loyalty - maybe there is a reason Samsung's products are selling like hot cakes and Apple is suing Samsung and not Asus?