Retired English Teacher
Which is better:
I would be grateful if you would... (do a favour)
I will be grateful if you would do... ( do a favour)
I prefer the second version because I think it sounds more positive/confident.
'Should', to me, tends to connote the sense of obligation.
I would use I would be grateful if you would- will sounds strange to me.
NOT A TEACHER
I am so grateful to you for teaching me the difference between:
I should/would be happy if you would visit me. = polite request.
I should/would be happy if you visited me. = If you visited me (you probably
are not going to), I should/would be happy.
(Now I have to edit my original post again. Don't want to mislead any learners
such as I.)