St. Petersburg is a 4.5 hour drive from Miami and, though different, has a lot of similarities. St. Pete is on the Gulf, Miami is on the Atlantic. St. Pete is full of tourists, as is Miami. The Port of Tampa (St. Pete/Tampa/& Clearwater are basically considered one area) services Central Florida and is doing a lot of business with Brazil; Port Miami services Central America and also does a lot of business with Brazil.
There are a lot of things to do in both St Pete and Miami, but I am a beach girl and spent the time I was in St Pete on the beach with my nephews.