What's the difference ...

One smells of vinegar the other doesn't ;-)

Sanitary silicone is generally "acetoxi cure" which goes off faster, and is usually high modulus, i.e. it's more rigid.

frame sealant is generally "neutral cure" which has better adhesion to plastics, and generally low modulus, so it has more give to deal with expansion and contraction.

Yep, the acetoxi cure stuff is really not what you want if the frames might be steel. Number of times I've seen cars that people have used bathroom sealant on...

Contains a fungicide...

Doesn't, and comes in a range of wood like colours. You may find the better frame sealants are neutral cure so that there is no acetic acid generated in curing that may stain some building materials.

I've generally preferred acrylic for outside for the last 30 yrs. I find silicone a pain to work with and it goes off in the tube after a couple of years IME. Silicone works for glass brick installation with plastic guides, but you need to finish off the visible joint with acrylic as the silicone leaves a film behind.