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Nothing. No difference. The sounds just slightly more polite. I wonder why? Perhaps it suggests that the listener might have only one chance, because s/he is so busy and important.
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There is a slight difference and it has shades of the difference between infinitive and gerund. It has to do with that word specificity, which I just love. Again some examples:
If you get a chance, I should be gratetful if you would just check this letter for me.
This suggests if you have a moment and there is no desperate rush.
If you get the chance, I would certainly recommend you visit the magnificent cathedral in the city centre.
This suggests that this is a special opportunity which shouldn’t be missed.
Many thanks for your immediate response. You know, there are situations when a question just pops up and you when you get a quick answer, you will memorize the information much faster too.[YSaerTTEW443543]