My rule WRT calibers is rather simple: Stick with past or present military cartridges, except for 22LR.
For handguns, of course, this basically means 9mm or .45Auto.
Why?
Because the latest 'fad' caliber may be 'cool' for a gun nut, but for someone who's just getting in to shooting, all it's gonna do is reduce the amount of rounds they fire due to higher cost & the ability to find ammo...
In general, get a 1911 or USP if you're going with .45, and I'd reccommend a Beretta 92 or HiPower for 9mm...
The 1911 is the 'classic' .45ACP weapon, so inclusion is somewhat self-explanatory....
As for why the USP as choice #2? Well, it offers the most versitility of the 'modern' .45s. You can use it DA/SA like a Beretta, or single-action 'cocked & locked' like a 1911 or Glock. It's available in multiple sizes, and the .45 is probably the 2nd most common autoloader cartridge besides 9mm...
With regard to the 9mm weapons, I pick the Beretta due to it's common adoption (US military, several PDs, etc...) and the availability of cheap & reliable ($9.99) 15rd magazines (Who wants a gun where full-capacity mags cost $40-100 each?).
Finally, the BHP is to 9mm what the 1911 is to .45ACP. Aside from being a direct 1911 descendant, it has a comparable military record to the 1911, serving the same role in FN-armed countries that the 1911 did here... Due to this history, mags are also commonly available...
That's my (long-ish) list...