I had always wanted to fly sailplanes but when I had the opportunity to
learn to fly, I opted for power since I lived in Mesa, not two miles
from Falcon field. I added on the glider rating two years later out at
Turf Soaring. The only advantage to this sequence wat that I was able
to get a lot out of the way in the power training, airspace, charts,
radio, etc. that I did not have to worry during glider training. While
I definitely learned more about "flying" getting my glider rating, I
did not feel that I needed to "unlearn" anything. It's great to have
both ratings, but you need to work to stay current in both. I have not
flown power in the last two years, as I have been spending all my time
(and money) on soaring. It is definitely more fun. My intention is to
fly power during the winter and soar during the summer.

If you can afford to do both, I doubt that the sequence is all that
important. It has more to do with your personal situation, time and
money.