Personally, I would always opt for a price reduction vs a seller’s credit because you will otherwise be paying property taxes for this credit. The only reason to request a credit instead of a price reduction of the same amount is if you don’t have enough financial resources after the down payment, closing costs, etc. to do the work and still have some wiggle room.
Depends on how much you’re talking about, but in some cases, the amounts can be substantial, and you’ll be paying taxes on that amount (plus 2% a year, compounded) for as long as you own the home.