If conditions under section 15(3)(b) are satisfied, it is better to issue credit-note with gst u/s 34. This is because once conditions under section 15(3)(b) are satisfied, value of your supply does not include your given discount.
You may still issue financial / commercial credit note, if recipient does not want to reverse ITC and you do not want to take re-credit of ITC. But, there may be a possibility of legal dispute at recipient-end for ITC availed against "discount" despite satisfaction of section 15(3)(b) (i.e. except not-reversal of ITC at end of recipient).
However, such issuance of "financial / commercial credit note without GST" (instead of credit note u/s 34 with gst) should NOT be used as mechanism to transfer hugely-accumulated ITC from end of supplier to recipient through deliberate planning / business structuring.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion or recommendation.