Agile: When to re-factor and when to extend while accruing technical debt? [duplicate]

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This question already has an answer here:

• Rule of thumb for cost vs. savings for code re-use [duplicate] 7 answers
• Agile estimation with tech-debt 3 answers

Consider the following scenario.

You currently have a feature set A you wish to extend to include feature set B. In the near feature there's a high possibility that you wish to extend this even further to include feature set C.

• Option 1) Cost to extend A -> B is minimal, but high maintenance costs.
• Option 2) Cost to refactor A and B to make more logical sense is moderate, but low maintenance costs.

Extending to include feature set C:

• Cost to extend refactored(A & B) -> C is minimal, again low maintenance costs.
• Cost to extend A,B -> C is high, (both to refactor and extend).

Assuming the inclusion of feature set C is beyond the scope of the current sprint, how do you go about choosing an option?

How does this change if the addition of feature set C is several (2 or more) sprints out?