9mm is the best option if you're learning and intending to frequently shoot a defensive pistol on a budget. I like CZ and Sig for metal framed guns, and feel most of the the polymer staples (Glock, XD/XDm, FN, SW M&P, etc) are on pretty equal footing. Simply getting a grip on as many possible will offer pretty solid insight into which fits you best.
The other practical options (probably in this order, IMO):
357mag- Ruger, S&W
45acp- various polymers, Sig P220, 1911
40sw- Similar makes/models as 9mm
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