If you have one, and only one 1911, I suggest a full-size or commander-size due to reliability. I would carry a 3" only as a back-up piece. This is in fact what I do.
My Kimber Ultra Eclipse was very unreliable, as in literally 90% unreliable when new, and even after the 2nd trip back to the company, it remained 70% unreliable even after far more than 1,000 rounds of all sorts of ammo. In all fairness, there are other Kimber owners whose Ultras have been "Ultra"-reliable.
My Colt New Agent is been completely reliable, but I have only fired around 150 rounds through it. So, I can't really claim that it has been thoroughly tested. After I have launched 1,000+ rounds through the New Agent, then I'll have a better idea of its true ability.
But, I do know that most 1911 owners will assert that the full-sized 1911 pistols are more reliable than the shorter 1911s. So, in closing, full-size or commander-size. Use a 3" for back-up.
Here are a couple pictures of me, putting my money where my mouth is. The first picture Colt Series 70 reissue (5"), with Colt New Agent (3"). The second picture is a Kimber Warrior (5") with a Colt New Agent (3"):
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