I wear mostly shorts and occasionally wear bibs. I don't feel any difference in comfort between the two, and since bibs are usually more expensive, I tend to buy mostly shorts.
I've used a variety of shorts; Pearl Izumi, Cannondale, Nashbar brand, etc.
I've never had any issue with them riding down or folding around the waist, my main issues have usually been learning which brands/models have the best padding, and which are the most comfortable, etc. Some designs will just fit you better than others and if you find a pair that you really like, watch for sales and try to buy several pairs....because if you have my luck the next time you need a pair they'll be discontinued....
Just keep in mind that sizes can vary a lot between brands, so if you shop on-line you will be out shipping costs if you have to return items....
Edited to comment on the OP's original question about tights: I wear non-bib non-padded tights in cold weather over my padded cycling shorts. I have two pairs; one is a Performance Bike house brand and one pair is a Nashbar house brand. No issues with either riding down, the only difference is the Peformance tights are heavier/thicker and are better for colder temps...I sometimes use them when I run my snow blower and it's really windy. They keep my legs warm and the snow doesn't stick to them like it does to bluejeans...so they get double use.
