Summer vs All season tires

A/S
1. Summer tires suck in the snow if you should get hit with an oddball snowfall
2. Summer tires are usually geared towards performance which the Prius does not need.

a/s

This is what I use and recommend They cost a little more but it's not hard to 100000 miles out of a set
Defender®
Passenger Car/Minivan
ALL SEASON TIRES
Michelin's best standard passenger tire , offering 90,000 miles of confident driving through exceptional safety, longevity and a quiet, comfortable ride.

Definitely all season, but since you have snows you can get a good handling performance all season. Look at reviews on tirerack.

Summer are biased toward dry traction at the expense of poor treadlife, harsher ride and more noise. Not only will then slightest snow strand you but they tend to get hard as a rock as soon as the temp hits 40f and get slippery.

I used to have real sumer tires on my RSX in my 20s thinking I was a hotshot. Never used more than a fraction of their capability on dry roads. I had to switch to the snows early...sometimes late October... To be sure I wouldn't get surprised by snow or ice.

If your looking for MPG and long life get a tire with solid shoulders or centers. The less ribs in the tread the better.