I have both bib cycling tights and bib tights (worn over regular cycling bibs) that I wear for training rides. Nothing under either and both work well. I have both Assos and Pearl Izumi and like them both. Not a big fan of Specialized and some other bibs that seem to wear out much faster and aren't as comfortable.

For all transportation riding though, at least up to maybe 25 miles each way, or much more, I wear regular jeans, khaki's, or slacks and ride a Dutch city bike with a fully enclosed chaincase so no problem with grease or anything. Bicycling should be simple and enjoyable. There's no need to complicate it with a gob of unneeded stuff.