Your call and SMS logs should not be affected since they're stored by your phone and not the SIM. If you want to play it safe download one of the SMS/Contact backup apps from the Store.
As for the service, it of course depends on where you live but having switched from Ma Bell to T-Mobile a few years ago I can tell you that you'll be relieved to see a stable bill every month. It's the only reason I left, it would fluctuate $20-30 every month and I would waste an hour of my life every month on the phone with CS to get it straight.
Good luck.