Powdered sugar as a mite treatment shows that the bees are interested in getting it out of the hive ASAP, not eating it. I don't think they will eat much of it when dusted, but if you are intending to feed them commercial powdered sugar in any form, be aware of the cornstarch content.
Dry sugar in the summer may mean less work for you but seems to me to be a lot more work for the bees. They will need to liquefy it to be able to store it. In the winter, condensation in the hive is usually enough. In the hot summer, wouldn't they have to bring in extra water beyond that which they normally need?
If they need to be fed, I'd try to find the time to make syrup. I can make 2 gallons in a few minutes using hot water from the tap and a paint mixer in my electric drill.
Wayne