pomprocker posted while i was typing, but since ive typed it aswell...
razor is right, but it really depends on what you want...
The $50 robot can use any of them
the 1st one has a range of 10cm to 80cm so your robot is effectively blind between 0cm and 10cm range
the 2nd one has a larger blind spot 0cm to 15cm but after 15cm it can see as far as 150cm
the 3rd one is only blind for the first 3cm but it can only see 30cm maximum
What would be most useful for what you want it to do? and what maximum or minimum distance constraints do you want to work with. (the edge finding algorythm will work with any of them).
You might be tempted to just get the one with the largest range but the pre made edge finding function might make it concentrate on walls rather than following you around in a smallish room. (you can of course enhance the algorithm yourself).