Originally Posted by FBinNY

Either is OK assuming your RD has sufficient chain take up capacity. Be aware that most mechanics fit chains as short as possible so a larger cassette may mean a new chain.

Use a gear chart, to determine which arrangement will give you the best selection of the mid-high gears where you'll do 90% of your riding, combined with an acceptable low.

In your shoes, I'd go for the granny solution, 24 or 26t if I expected to use it only as a bailout and stay in the 39t most of the time. In that scenario, you'd have the benefit of the tighter cassette for smoother shifting, and tighter middle gearing, and while upshifting out of the granny may be crappy, it won't be so frequent as to be an issue.