Many people say the leather ones cannot be used for extended sessions, This is a concern for me.
Thanks in advance.
They are both more comfortable and cooler to wear. And IME, no, they don't affect sound.
FWIW I've worn my LCD's for hours and hours and hours LCD-3 22 hours straight, LCD-2 12 hours straight.
I find no major gripes with the comfort of the leather, but do prefer the leather free pads.
I agree with paradoxper, the "vegan" pads are very comfortable and can be worn for extended periods without serious discomfort. I hear no significant difference in sound quality between leather and vegan pads on my LCD-2.1s, although I was told by Sankar when I order a pair of "vegan" pads to replace my leather pads, that some people heard a slight loss of bass with "vegan" pads, but more openness.) I don't hear a loss of bass, but I do hear a slightly more spacious sound. I'm pleased with the change.
Leather bad. Vegan good.
Not difficult at all. Awhile back, I posted a pictoral step by step: http://www.head-fi.org/t/586157/audeze-lcd-2-vegan-earpads-feedback-installation-mini-review
Audeze also has a "howto" video on their website: http://audeze.com/padschange
All you need is a little care and patience.