This looks like an old thread resurrected and the conversation is mostly moot, but as a general rule of thumb you don't want to oversize the fan any more than you need to. It's like a furnace or anything. Properly sized is more efficient and more isn't always better.
The "standard" formula I've seen is to replace the air 8 times/hour. You have 60sf * 8'? That's 480cf to change 8 times / 60 minutes = 64 cfm fan. I would have suggested the 80 unless you have a heat exchanger or enjoy high utility bills
You also might need to enlarge the gap at the bottom of the bathroom door or you'll just be taxing the fan and making it die (and warble miserably) sooner trying to pull 80cfm through the usual 1" gap (< 0.25sf) under the door!
Size it right and use a timer switch on the fan... My 0.02.
Or, let it die sooner and buy another American made fan probably filled with Chinese parts.