Mountain Biking - Cassette vs Freewheel

I know cassettes are what come on most bikes these days but do they have a huge advantage over a freewheel? I was just curious because Jamis bikes use them (at least the CC 2.0 and the Durangos). All I've really read about them is that cassettes are newer while freewheels were supposedly pretty interchangeable. In short, would it be wise to only look at something with a cassette and if a bike has a freewheel can you simply change the assembly over to a cassette or is it more complicated than that?

Whut???? A cassette and a freewheel cannot be interchanged.

Well that's kind of what I was asking. If you could switch out the assembly, but after reading some it seems you need a new hub? My question was basically can you switch from one to the other, or rather how do you do it? And also is a cassette better to have.

you cannot swap. I prefer cassette because of more optiions.

Little vague there Ronin. What 'options' does it give exactly?

Cassette gives you 7(with a spacer),8,or 9.Plus you can convert to SS if you want. As far as I know nobody makes 8 speed,or 9 speed freewheels. Freewheel is obsolete more or less. cassette and freewheel are different hubs.it would be hard to find a modern hub in freewheel.

Ah ok. That's more helpful, thanks. I think the Jamis bikes are 8 speed but my internet is slow at home so I'll check later. I was just wondering if it'd be a huge disadvantage and it would save me a trip if I didn't want to look at Jamis bikes.

Cassettes, at least with most hub designs, allow the bearings to be placed closer to the ends of the axle than a freewheel would allow. This makes bent axles much less likely.