Paint Prep

I am painting some interior bedrooms and such. The previous home owners used gloss paint on the walls and in one room the same gloss paint on the ceiling. I'm trying to decide a couple of things:

1) There are deglossers/cleaners available that apply and wipe off as prep for the new paint. Any reason to not use one of these vs sanding? Are there any particular kinds or chemical makeup that do the best job of this? The deglossers seems easy to use (with proper ventilation) and sanding seems like a messy workload. I have some friends who've used the deglossers and they were pleased.

2) The colors in the rooms are a bit darker so I'll need two coats of new paint to cover it. They are nothing extreme, so just average I would say. My thinking was that if I have to do two coats of paint - why not make coat #1 a primer/sealer. Similar amount of work and cost so its really no impact. Are there any pros/cons to 2 coats of paint vs 1 coat primer/sealer and then

1 coat paint?