http://www.woollyhatshop.com/view_product.php?id=1528

http://www.woollyhatshop.com/view_product.php?id=1529

All I can see is a different spring type – top or low. How does that make a difference?

Its to use on a shimano xt 11-32 cassette.

first one is a 'normal' rear mech. Second is a 'rapid rise' rear mech.

Rapid rise rear mech mechs work the opposite way to normal one's.
Small trigger to move up the cassette big trigger to move down the cassette.

Alright, cheers.

One is rapid rise, one is "normal". Alters the shifting by reversing it IIRC

Always check on shimano dot com for compatibility queries.

Rapid rise rules by the way. I run both types on two bikes but only the rapid rise gves me absolutely flawless downshifts up steep hills under power.