^^^^^^^ what basic said ^^^^^^^^ solid coverage in the area where you're going to be using the phone the most is EXTREMELY important. There is NO more frustrating thing than talking to someone and just ... nothing.
Let's see, we've had verizon, sprint, nextel and a motorola pager.
Shittiest service, IMO, nextel and sprint, it's nearly a dead heat. Nextel likes to drop out a lot in places you'd NEVER expect. Sprint has big blank spots but pretty good coverage in populated areas.
Most consistent coverage is verizon, motorola is right up there, too.
450 minutes and unlimited nights and weekends means any calls you make OTHER than your unlimited times will count against those 450 minutes. So let's say your nights start at 9:00 p.m., and run until 6:00 a.m. You can make and receive constant calls from 9:01 p.m. to 5:59 a.m. and it doesn't matter. HOWEVER, any calls you make from 6:01 a.m. until 8:59 p.m. will be deducted from your "free" account and when you have used that cell phone for 451 minutes you start racking up charges at the rate of anywhere from $0.50 a minute to (more likely) $1.50 a minute. Unlimited weekends means you can use the phone as much as you would like from 9:01 p.m. friday night until 5:59 a.m. Monday morning and it will not cost you in minutes.
The thing you need to keep in mind: 450 minutes means you can't use that telephone for more than 15 or 20 minutes total during the day on weekdays without running into extra charges.