What else do I get with Cat 6 beyond future proofing my wiring?

We are about to put in the wiring in an expansion of our office building and the vendor is proposing we use Cat 6 cabling because it is "faster." Gigabit works fine over Cat 5e which is what our existing building has run throughout it and according to Wikipedia we would need Cat 6a to be able to actually support 10 GB though it may work over shorter distances with Cat 6 (how long is shorter distances exactly?).

People talk about the increased speed/bandwidth of Cat 6 cable over Cat 5e but I believe what they are talking about is that Cat 6 supports 250 MHz vs Cat 5e support for 100 MHz. My understanding is that this does not at all translate into any difference in MBps which is what I normally think of in terms of speed.

If I setup a test with two computers at a distance of 100 ft that transfer data between each other using Cat 6 and then again using Cat 5e I don't believe I would see the time to transfer a 10GB file vary at all due to the cable used.

To further confuse the issue variance by MHz doesn't seem to be a reliable measurement of performance even when comparing cabling as the Cat 5e 350 MHz debacle illustrated.

I will be the first one to admit that I am not expert in this area but I am struggling not to make this decision based on what I see as cable suppliers use of fear mongering to convince me to purchase something that provides us no real benefit other than protecting myself by making the choice that no one will question in the future.

Are there other benefits that I am not considering or things that I have misunderstood which make a compelling case for Cat 6 instead of Cat 5e?

I'm no expert either but I'm on board with what you've stated in your question. My opinion is that the vendor is giving you a snow job in order to make more money off of the job by selling you CAT6 cable. My recommendation is that unless you plan on moving to 10GbE (CAT6a cable needed) in the near future then stick with CAT5e. GbE (1000BASE-T) will happily run on CAT5e or CAT5 cable.

Read the wikipedia articles on this stuff.

There is no real advantage of cat6 over cat5e. Specifically, both can run gig ethernet the same distances and when you go to 10gig ethernet, neither is 100% guaranteed to work.

If you want to be able to run 10gig ethernet over copper, you need a 100% cat6A cable plant, not just cat6. With cat6, you can run up to 55meters "in a favorable alien crosstalk environment" which means you don't have lots of cables right next to each other. I've run 10gig over cat5e without problems on 100foot runs.

Cat6a is hugely expensive and difficult to work with. Cat6 is almost as expensive and nearly as hard to work with (thicker, harder to bend, etc)

Whatever you do, make 100% sure that the installation is tested with proper equipment after it is installed. Just because all the parts are marked as "cat5e" or "cat6" or "cat7a" or whatever, doesn't mean anything. It all needs to be put together correctly for it to actually perform to the specified ratings.