A few milliamps can kill and a 12A load is max for a 15A circuit. Where's the data that shows a properly installed 15A circuit is safer?Use 12 ga if you insist. It is not as safe as 15 amp circuits.
I'd run #12 with (some 15A breakers. Forget about the high cost of copper, it's the AFCI breakers, if they are required (yet) in your area that will break the bank.
By code, one 20A fine for running just the wall outlets in both baths. However, forget using two hair dryers, etc...
As for tools in a shop one could run these together:
- 2 hp table saw
- dust collector
- air compressor (could start)
- air conditioner
May not be more than 50A, but getting close. Add to that several hundred feet of undersized feed from the main panel and the lights start to dim... The cost of a 100A vs a 50A panel is nothing compared to the installation costs...