I'm a french Canadian and only ISO keyboards are available out there. I wanted a ISO mechanical keyboard too, but I ended up with an ANSI one.
My point of view*:
-I press enter more often that I press the key to the left of the enter (on an ISO layout). On a ANSI layout, the enter key is easier to reach.
-I'd use the left shift key more often that I would use the 105th key (I just don't use it). The ANSI layout makes the left shift key easier to reach, because it is bigger.
-The ANSI layout is more comfortable and is nicer, because every key uses only one row.
-More keys doesn't mean faster, it just means more comfusing.
I didn't know if I'd like the ANSI layout, I was used to the ISO one. Turns out I prefer the ANSI one because it's more comfortable and makes touch typing easier. I ordered a ANSI keyboard for my netbook because it has an ISO one since I'm a french Canadian, just to show you how I like it.
It takes a little time to adapt, but it is worth it.
Oh, also, most of mechanical keyboards use the ANSI layout.
It is easier to adapt to the ANSI layout than looking for a ISO mechanical keyboard ;p
EDIT*: try the layout on a cheap keyboard before buying something, you'll see if you like it or not