If I had to guess, I'd say that the 5 1/2" barrels are the most popular. Why? 'Cause folks can't make up their minds on barrel length and that's the choice in the middle
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. Some shooters like the shorter barrels because they feel they can clear leather faster. I like the 7 1/2" barrel because I find that the additional barrel length makes the sights a little bit easier to pick up for these old eyes.
My 7 1/2" barrels are in .357. Little hole in a long barrel = extra weight hanging out there. I don't feel the difference, but my wife says she can't shoot my guns because they're too heavy; she shoots .357 4 7/8" barrel pistols.
Your best bet is to go to a shoot and talk to the shooters. Ask them the same questions about barrel length, and you'll find yourself trying all kinds of different pistols; cowboy shooters are like that.
The bottom line is shoot what feels best to you!