It isn't all that much of a change, and adding a couple pairs of bike shorts can really help you make the jump from weekend to serious cyclist, and your butt will thank you for it.

Levels

In general, each bike shop you visit will most likely offer bike shorts in 3 ranges. Good, Better, and Best. Yes you absolutely should try them on. Internet shopping for these generally results in cheap shorts and disaster. The low end will probably be good enough for 25 miles. The middle would work for 25-50, and the best will work all day. Aside from material quality and finish work associated with a larger price tag, those are basically the differences.

Brands

These are a lot of nice brands of shorts, but generally speaking there are a couple of brands I can recommend. On the value end, Sportful and Pearl Izumi. For the middle tier there is Louis Garneau, and Castelli is one of the priciest offerings. Don't let this fool you though. The offerings from LG in my opinion are the best on the US or Canadian market without a second thought. They are so much better than Castelli and cost much much less.

Bib vs Shorts

In the upper ranges you will have to decide to keep with shorts or to use bibs. Bibs are weird, but I honestly think these are the best options for anyone. The difference between these and the regular shorts is that bibs have the straps that go over the shoulders. These straps hold them up instead of making you rely on an elastic waistband. The waistband can slide down and applies pressure to your waist while riding. The straps keep them in place firmly. No shifting, twisting, sliding, or falling here. Bibs will feel tight when you stand up, and that is normal. In a cycling position the pressure goes away.