I'm going to spend a day in Tongariro on August 11th. The big debate is to do a day of skiing or do the Tongariro crossing hike. I understand the weather can be bad and proper equipment is needed. Are there any guided tours? Any places that will rent the proper equipment? Or am I better off just spending the day on the slopes?

" The Tongariro Crossing can be done on a beautiful winter’s day, but for your safety you must be accompanied with a guide experienced in our mountains.  Weather on the Tongariro Crossing is subject to Alpine conditions and can be extreme, particularly on the “Red Crater” ridge. We encounter snow, ice, and often strong winds.  Weather Forecasts are used as our initial guide and then your leader makes the decisions based on the actual conditions experienced. "

http://www.tongarirocrossing.org.nz

Skiing in miserable conditions is just as bad as attempting the crossing. Lots of lower level tramps in the area - which are quite delightful and safer!
It all depends on your tramping experience.