Honestly, you need both for maximum efficiency. But I'm voting on technique due to my own experience with the serve.
I'm honestly a pretty weak guy. My upper body strength is really low, and since I stopped exercising my core and my legs, they have gotten weaker (bad habits built up after I got a concussion last year). So relying on power would be a mess for me.
Even though my muscles have gotten weaker, though, my serve technique got better after practicing that and my serve sped up considerably.
That being said, power is important. Technique, however, is more important.
In my opinion, technique should be the starting point. It will lend its way to more power, but you should still exercise properly to increase your service speed and weight, too. Mainly relying on power will limit how good your serve can be, as will only focusing on technique. You need both. But without technique, your serve will be less accurate and a good second serve may be non-existent.
Work on both aspects, but remember the importance and limitations of each.
After all, who was a better server in his prime, Tsonga or Federer?