The differences are the type of coverage, where you funnel the receivers, if
you take an inside out stance or try to keep them on the sideline. A safety
is often in an under or over coverage much more frequently too...and the
size...and that depends on the scheme, how much gap responsibility will he
have.
in my unqualified armchair football mind, i always think that the FS
in the jets style of system should be a 3rd CB any way and the SS
should be more like a very fast LB.
Most of the time that is about right...and the FS needs to be a little
tougher, may not have to be as fast.
In high school I was a safety and went the other way, got shifted to
CB when the guy there got hurt.
I was used to reading the play in front of me and taking angles, and
had nothing like the quickness or backpeddle ability a CB needs. I
suggested that maybe this was not such a good idea, but it's team
first and the coach is always right, right?
The other team saw this immediately, put their best receiver on me and
he came at me three straight passes.
The first one was a sideline pattern, I was bigger than him, and he
sorta got pushed out of bounds. The ref didn't notice, :-)
The second one he cut in, got ahead of me, got his hands on the ball
but dropped it, though a knee in his back might have had something to
do with that. The ref didn't notice. :-)
Now this guy is really pissed. The third one he comes straight at me,
and I mean *straight*. I backpeddle as fast as I can but I'm going
"whoa! whoa!" in my head. This was one of those olden-days sports
fields that served multiple sports, there was a baseball diamond on
it, and this guy drives me backpeddling right onto it. Not having
eyes in the back of my head I didn't see the dirt and as soon as my
feet hit it I went right down flat on my back. As I looked up the guy
was still running that same damned straight line now with the ball in
his hands disappearing to the far end of the field.
Next play I was back at safety. :-)