Oops, just ordered the wrong cassette. Got a 11-32 isntead of an 11-34.
 

Think the differnece is worth cancelling the order for, and ordering the other cassette, or are they similar enough that I won't even notice the difference between 32 and 34 on the bike?

Well, I think about those times I need a 34 and they're pretty rare but when I do need it, I REALLY need it. If you're expecting a 34 and you go to grab for it only to find that the lowest you've got is a 32, you might be a little bit disappointed... and that's an understatement. If it were me, I'd cancel and reorder.

I changed the order.

Wimp.

I just decided to go from 34 to 32, partly because I rarely find myself really needing the 34 and it is far enough to keep the front end on the ground seated when really using a 34. By the time I hit that steep a climb, I'll just spin out the 32, drop a gear and stand. Think more of a roadie style, and I find it easier on my knees.