The smart trainer has had the most effect on performance for me, but the powermeter is a close second. Just realize a powermeter is only a sensor, so you have to have some idea what to do with the numbers and how to analyze the data.

Monday I went for a "training" ride and was able to keep my power in a specific range for a couple of long intervals. If I were riding by feel I would have probably been blowing myself up during the beginning by pushing too hard and then fading by the end of it. That's one instance where the PM is useful. Keeping track of TSS, CTL, and fatigue is also helpful.