Does it make sense to stay at union square and travel to and fro everyday to San mateo for a workshop?

Yes, that can work, take BART each day to San Mateo if you don't have a car with you.

That's quite the commute. Also, union square is more expensive. It makes more sense to Bart to Union Square for entertainment if your workshop is in san mateo.

Yes, Bart should be able to get you to San Mateo, traffic free!

BART to San Mateo?¿

Yes...