Yes, the SV and SL get the time from elsewhere. I'm not sure about the S, as it has no telematics and I don't think it has satellite radio...?
You can get a cheaper AGM* battery - it doesn't have to be a Yellow Top. Just try to determine that it's a real AGM battery, and not just labeled as one. I think that one way to tell is to look to see if it has an Amp Hour (AH) rating as well as a Cold Cranking Amps (CCA) rating. AGM batteries are designed for deeper discharges than are starting batteries.
* "Absorbed Glass Mat"