Inside or outside: which windows get dirtier?

Outside gets the weather: dust, pollen, rain spots, sprinkler minerals and whatever the wind carried. It is the side that visibly degrades, and it usually needs cleaning more often.

Inside gets cooking film, dust, fingerprints, pets and condensation marks. It builds more slowly and more evenly, which is why people often do not notice it until the outside has been done.

If the budget only stretches to one, do the outside. Just be aware that a freshly cleaned outside makes an untouched inside very obvious.