The difference between freewheels and cassettes is huge. If you buy a bike with a cassettes it may or may not be a high quality. However if your bicycle has a freewheel it is low end. Freewheels dominated the market 30-40 years ago, however they were supplanted by cassettes over 20 years ago.

So why did cassettes take over the market? Freewheels do not move in a perfect circle which makes rear derailleur adjustment more difficult ( especially with a multi chainring setup ) and it adds resistance to the drivetrain system. Cassettes on the other hand move in a perfect circle and do not add unnecessary resistance. Their popularity has grown dramatically since their inception 30 years ago.

In the past twenty years cassettes have expanded from 7sp to 11sp and everything everything in between. Additionally there are different cog configurations such as 11-42 ( or 11-32 ) is popular for mountain bikes while 11-21( or 11-25 ) is popular for roadies. Freewheels have very limited options, there are about 10 different 7sp freewhel options in the USA market, and even less for 6sp freewheel setups.

Stick with cassettes, you'll like them better.