Our hotel has offered to reserve for these 2 museums at a price of 15 Euro per person, per museum. Am I better off reserving myself via telephone?

yes. they are marking it up considerably.

I don't know, sounds expensive to me. Are you sure it is per person per museum?

That does sound high. The telephone reservation system is actually quite easy.

We paid 12 euro per person with telephone reservations in July. I'll see if I can find the telephone reservation info for you, and post it here.

From another poster.

The easiest and cheapest way is to call Florence 011 (U.S. international access code) 39 (Italy's country code) then 055-294-883 8:30-18:30 M-F and 8:30-12:00 Sat. Florence time. You will get an English speaking operator and in 2-3 minutes YOU CAN RESERVE FOR BOTH. This is through the reservation service at the Uffizi and costs beyond the normal entry fee only about 1.60 euro for the service. This is MUCH cheaper than the commercial booking services.