As far as .45 ACP, it's a matter of whichever one you like best and fits your hands. 1911 from any quality manufacturer is a great start. Then you've got plastic fantastic options from Glock, S&W, SA, Ruger, Kahr, FNH, H&K, etc. All of them are better than good, as are other all-metal options from Sig, CZ, etc. You've literally got dozens of options to choose from.
As far as AR pistols go, that's a completely different subject. You'll have to decide for yourself what you want a pistol for, be it SD/HD, possible CCW, range toy, or some combo.
If you're looking for a normal pistol, for presumably wide range of uses , then absolutly a .45acp ( of the listed choices ) .
There are more quality options in .45acp than you can shake a stick at. If you had an already set preference in operating system ( SA, DA/SA , DAO , Safe Action , etc) , or general size range we cpuld try to point you.
Failing that , expose yourself to as many different ones as possable , preferably firing them. Eventually at least one will particularly fit you well , and/or be able to shoot it particularil well.
1. To me, the 5.7 ammo is waaaaaaaay too expensive. (Personally, I have no desire to own one, or even shoot one.)
2. AR pistol - I'd rather have the 'carbine'.
3. Get the .45 acp. Hold/fondle as many different types/kinds as you can, and see which one(s) fit YOU.
The Glock G21 and the G30, plus the G30SF are great pistols.
The S&W M&P .45 is a great platform, too.
1911 - There are more styles and manufacturers than I can name.
I never understood the AR pistol thing much, short barreled AR's throw fireballs and not having a stock limits accuracy. I would just get a carbine for sure and be done.
The 5.7 X 28MM is a neat little round, but boy is it expensive! If you bought a pistol and the PS90 that shoots it, you could buy in large quantities to hopefully save money?
I personally would get a 45 ACP and be done. Great round, tried and true.
I have an FN 5.7. Ammo is a bit pricey in general. Like to shoot it - it's fun...BUT, I also have an FN 45, and LOVE it. If I could find a S&W M&P 45, I would probably like it a bit better than my FN 45 but I won't know until it happens.....:-)