For gearing changes, either one should work and I don't find a lot of difference in the effort either way. Changing the rear actually allows finer tuning of what you're after though. As a rule of thumb, a one tooth change on the front will be equal to about a three tooth change on the rear as far as the ratios are concerned, so changing the rear allows you to choose smaller steps toward your goal if you so desire.
The other consideration is that a smaller front sprocket will induce a little bit more wear on the chain. It's not major, but there is a difference.
Edit: Stainless answered while I was typing.
"Fast is fine, but accuracy is everything"--Wyatt Earp
'13 Yamaha Super Tenere.
'04 Sportster XL1200R