Within reason
I think that most police officers should be able to carry what they want within certain parameters. I agree that .38 Spcl/9mm is the minimum - that would be one parameter. If I were in a large metropolitan area (I live near Houston, so I could see this applying here), I would agree also that the Magnums are probably out - the greater potential for excessive penetration (although that can probably be cured with rapid-expanding hollowpoints, but then there is the legal threat of having such ammo).
However, when I was up in East Texas, I wouldn't blame cops for wanting to carry .357 Magnums. They might be miles and miles from backup and need heavier firepower. I know...pack a rifle or shotgun. Lotta good it does a LEO if his AR15 or Mossberg 500 is in his car, 25, 50 or 75 feet away.
Some guns are out. I don't think a SA revolver has much of a place anymore; at least, in semi urban to urban areas. Maybe out west where bigger, heavier calibers (see above) are handy. A Desert Eagle is just asking for trouble for a cop.
If a cop can show proficiency in a reasonable sidearm, let him (or her) pack it. If he wants a 1911 and can handle it through training, so be it. If it's a Glock, more power to you. If it's the M&P, yippie-kay-yae-aye. If its a .357, go John Wayne!
These folks put it on the line for us each day. It's a shame some departments require our finest to carry 2nd best because it's cheap, someone's cousin knows a guy, or someone once heard something that someone said XXX style/brand/caliber isn't good.
My $.02 worth...refunds on request...
Q