I imagine they're both worse than good old exponential smoothing or SARIMAX.
And exponentially higher cost for ML models.
I also think it's a dead end to try to have foundation models for "time series" - it's a class of data! Like when people tried to have foundation models for any general graph type.
You could make foundation models for data within that type - eg. meteorological time series, or social network graphs. But for the abstract class type it seems like a dead end.