RonBee is right that 'should' is not used much in NaE for this type of situation.
But for BrE, I'd say that 'should' is the more polite collocation of the two but that 'would' is also used in this fashion.
Googled, Uk sites only, we get,
Results 1 - 10 of about 919 for "I should be very grateful if".
Results 1 - 10 of about 44,100 for "I would be very grateful if".
Of course, when we do a Google.com search, 'would' blows the doors off 'should' for frequency of use.
Results 1 - 10 of about 208,000 English pages for "I would be very grateful if"
Results 1 - 10 of about 924 English pages for "I should be very grateful if".