'Will have to" is used for making a promise to do something in future. It's used in First Conditional where the condition is possible to fulfill in future :
If you want a bigger screen, you will have to spend more money.
But 'would have to' is used to be more polite to make some hypothetical statement. It is used in 2nd conditional where it denotes an unreal possibility, i.e., a condition in theory possible to fulfil :
If you wanted a bigger screen, you would have to spend more money.
What if you want a bigger screen?
In that case, the above-mentioned both conditionals are possible. But in my opinion, the 2nd conditional sounds more polite.