I just use fast for everything. The slow isn't that much slower, and the fast is slow enough to allow plenty of time for glue-ups. Just today I glued up a 5x12x4"deep box with dovetailed joints with 205 West System- wetted out with one ounce of glue, mixed a fresh ounce and thickened it, applied it then clamped it up. It was still liquidous when I got around to cleaning up the squeeze-out.
The thing with any epoxy is it'll "kick over" (set up) quicker in large quantities, or in deep quantities- deep meaning, say, an ounce of it in a dixie cup will harden waaaay quicker than if that same ounce were spread out thinly on a pallet. Mix it, spread it out, and take your time getting the glue-up done right. (I'm not advocating a leisurely approach to epoxy glue-ups though. Have all your tools and clamps laid out ready to go, have done a dry clamping run ahead of mixing the glue, and don't allow any interruptions. A steady pace wins the race.)
We used the 209 "Tropical" hardener in the boat shop on really nasty hot summer afternoons, outdoors under a blazing sun. Even then, if it was normal smallish repair/glue-up, the fast stuff was ok- but you had to shake your butt and git 'er done!
