The calculation for SBI DC rewards isn't accurate. Let me explain:
You will earn 1.25%, but only up to 1,000 points (this includes the 5x rewards). This means that for any spending above ₹20,000 (as a safe amount), you'll only receive 0.25% rewards instead of 1.25%. I'm not sure about the IDFC reward system, so I can't say anything on that.
For an amount of ₹20,000, you'll earn 200 base points and 800 accelerated points, totaling 1,000 points. That's it for SBI campaign.
In your case, Case 1 seems best if everything goes right with IDFC (I've read somewhere, people are not getting points with IDFC, or getting it delayed after emails/calls)