I chose my first guns based on a couple of ways. First was based on real steel guns I'd fired before. Based on that, I knew the Armalite family was something I was very comfortable with, hence my choice of the CA M15 rifle as my first real AEG. It's a design that I find very ergonomic and I'm comfortable using. My size allows me to easily handle a full-size rifle. And now I also have the C8A2 and HK416 and I love them all.
And same for my first GBB. I fired several different models of real steel pistols and revolvers over the years, so I already knew my favourite was the Beretta 92 FS / M9, so it was a no brainer to go that route.