A good pair of cycling bibs / bib shorts is the most important part of every cyclist's wardrobe. Here's a short list why you should wear bibs:
With shoulder straps to hold them in place and a pad to prevent chafing, cycling bib shorts made from a Lycra-blend fabric are very comfortable for long rides.
In general, more expensive shorts are better because of the quality. They are more comfortable for longer rides, but there are some exceptions to the rule.
Winter cycling bib shorts are made from thicker, warmer fabrics, often with water-repellency built in.