front derailleur : best to get get a 34.9mm or 31.8/34.9mm

I have a 2008 Trek 6300 post size is 34.9MM I know both will work, but I didn't know if there was any advantage in using one size over the other. I'm looking to buy a Shimano XT M771 Conventional 9sp Front Mech. Thanks in advance for any help.
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