Definately hire a car. The best way in the Winter is north on I-5, then Ca 99 to Ca 41 then east to the park. There is another entrance off US 395, but in the Winter many of the roads in that part of the park are closed due to snow.
Hire a car. You ll get there faster and have the freedom to choose which sites you want to see. Make sure you get away from the valley - very touristy, and, while the best sites are seen by hiking, a car will get you to most of them.