I assume that this is is the ISO one -
Show Image
(http://www.preater.com/modelm/images/model-m-front-large1.jpg)
And this is the ANSI one -
Show Image
(http://upload.wikimedia.org/wikipedia/commons/5/57/ModelM.jpg)
The most common one is probably the ISO one due to the amount of countries that use it. I prefer the ANSI one myself.
Around here in the Netherlands, you definitely see the "AT layout" much more than the ISO (which is really rare) and ANSI being by far the most popular in the last decade - except for the macbook's "Euro" keyboard.
BEST ENTER KEY EVER!!!
Show Image
(http://stuff.silverorange.com/images/ibm-keyboard.jpg)
Either that or just a nice good old dark grey enter key.
I prefer the regular horizontal enter/return key like the one above. I also dig the lightly greyed out ones.
I guess It's because I am used to them or maybe I just don't like it when there is a different enter key shape which leads to other keys changing location to compensate.