First off, don't get confused between a spot attachment and a fresnel attachment, or between a hotlight fresnel and a flash fresnel...
A spotlight attachment produces a very different type of light from a fresnel - I know because I have and use both. I have both the Elinchrom and Lencarta spotlight attachments. The Elinchrom one has a zoom adjustment, the Lencarta one doesn't. The Elinchrom one needs a large studio because it won't focus a sharp image unless it has a lot of space.
A spotlight attachment is a focussed light, normally used with gobos (cookies in americanese) and the quality of light is (should be) perfectly even.
A fresnel is entirely different, with a bright centre that tapes off towards the edges (some are adjustable, some aren't). The Lencarta one (which is out of stock but which I'm hoping will be back in stock in about a month) is adjusted by moving the flash head nearer or further away. The Bron one has an adjustable iris, both produce a very similar effect. Bowens make one too, AFAIK it isn't adjustable but I may be wrong about that. Elinchrom do one too, I haven't seen that one either, but in any event it's tiny and I would be very surprised if it was any use for people photography.
The fresnels traditionally used in theatre and movie lighting are of course continuous, not flash. I remember a discussion on another forum about this, a couple of people who had them said that they prefered the light quality from continuous fresnels, but I don't think there's much in it and it's probably just down to personal preference.
One thing you should seriously consider is the amount of light lost when using a focussing spotlight (if you really do want a focussing spotlight) - around 3 stops. No problem with flash, but you'd need a very slow shutter speed or a very powerful light if you want to photograph people with a continuous light that's going through a spotlight, or come to that through a fresnel spot.
Hope this helps.