Which Macbook Pro USB port should I use to charge an iPad?

Actually, your Macbook CAN provide extra current for apple products. Your screendump indicates that it is charging with 500mA + 1600mA of "Extra Operating Current" which makes a total of 2.1Amps (same as the iPad charger).

See here for a comparison - http://www.hardwarezone.com.sg/feature-effective-charging-apple-ipad-2012

You can use either port, the limitation is that only one USB port can provide this higher power level. If you try and charge two iPads at the same time, only one will get provided with the higher power level, and the decision as to which is made on a first-come-first-served basis.

Edit to Add: I forgot to mention that the article says Macs with more than two USB ports may be capable of powering multiple ports (but not necessarily all) at this higher power level. Unfortunately they don't give any further details.

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