if you're doing everything on your own, then the chainring option should be cheaper.
It is just that changing the smallest ring requires the crank to be removed and put back on again, and that's easier to screw up than a cassette swap.


$25 for cassette, $10 for lockring tool, $20 for chainwhip
vs.
$10 for 28T chainring, $20 for crank extractor

most likely you already have the hex keys that fit the crank bolts.