Small Office Server - Processor Options

If your budget will allow it, the Xeon processor will be somewhat faster at printing. I don't think it will make a lot of difference to file sharing.

If the above is ALL they want, the Core i3 3.3Ghz will work.

Customers have a habit of adding things, so try to fit the Xeon into your budget if you can. Make sure you get lots of memory.

... Thinkpads_User

I disagree - if the purpose is file and print, I wouldn't expect ANY NOTICEABLE DIFFERENCE for a network of your size.  Indeed, I would say disk would be more important for printing since the server spools the print job and then feeds it to the printer... the faster the disk the faster the spooling/printing.

I think the better the CPU the longer the server can last, but it's really not that critical.

Not sure what version of server you're looking at but you might want to consider Server 2012 Essentials or Server 2012 Foundation Edition.  Essentials can do workstation backup and doesn't require CALs for up to 25 users; Foundation doesn't do workstation backup but doesn't need CALs for up to 15 users (though it cannot grow beyond 15 users - only Essentials can grow beyond 25 users).

I'd use a Xeon personally just because it's intended for that kind of use. As said above, the disk will be very important in making sure things run smoothly for your customer. It might be worth running some performance monitoring on physical disk queues and average queues to see whether there is a bottleneck there currently. Then you'll know whether or not you need faster disk configuration going forward.