It does not appear that you can get All Season Tires with National or a "winter package"

All I can see on their website is Roof racks?

Reason that I bring this up.

The MOST important safety feature you can have is Snow tires or winter tires. You will NOT find proper Winter tires on any rental in the USA. But you will find All Weather Tires which have a M&S (Mud and Snow) stamp on the side of the tires. This will allow you to skip using chains for the majority of times when driving.

There are three chain requirements for driving in Colorado and California

1. No restrictions
2. Chains on ALL vehicles except 4WD with Snow Tires (M&S tires are acceptable) - This is a fairly commend restriction.
3. Chains on ALL vehicles - Snow is DUMPING but not that common

Pricing out a 2 week rental.
National - Standard SUV - With All insurances $2106
Hertz - Standard SUV - Which will be (4WD/AWD) With Sirius Satellite Radio, GPS Navigation, All Season Tires, Ice Scraper & Roof Racks AND All insurances $1500 - That is also with no discounts, and you can easily get 5-10% off that with various discounts.

I fail to see why you would go with National for more $$$, chance of not being a 4WD and no all season tires.

Where are you going to be skiing ? I am doing Beaver Creek and Breckenridge and Vail this year