i bike everywhere in los angeles and i use a old crusty beach cruiser i got at a garage sale 4 years ago for $20.
to get the most exercise i suggest buying a cheap, heavy bike at a garage sale/craigslist and it will take you a lot of energy to get places. the trendy, expensive fixed gear bikes are light so you can get farther easier, but they also have that thing where you have to be pedaling constantly.
so i’m assuming on average the amount of “exercise” you get is the same, but the first one is cheaper and less trendy. and cheaper.
other cons of fixed gear bikes:
*worry about it getting stolen
*drivers assume you are a pro and are more aggressive on the road
*expensive