In my opinion, as an instructor who has worked at a busy professional firearms training school for over a decade:
1) Beginning semi-auto shooters tend to learn best and fastest on mid-size, mid-weight DAO 9mm guns.
2) Beginning revolver shooters tend to learn best and fastest with a mid-size steel revolver in .38 Spl.
3) People who start on .40 S&W almost always develop a deep-seated flinch problem that takes a lot of effort (and ammo) to overcome.
4) People who start on a .45 ACP tend to have less trouble with flinch than those who start on a .40, and more trouble with speed than those who start on a 9mm.
5) People who start with tiny little lightweight guns often develop multiple poor shooting habits, and often have a very difficult time internalizing muzzle control.
There are exceptions to every single one of these rules of thumb. (The race isn't always to the swift, nor the battle to the strong -- but that's the way to bet, especially when $500 or more, plus your life, is riding on it.)
My personal website: Cornered Cat