Both matter. You are unlikely to be able to photosynthetically saturate your plants, just because it requires too much light and too much nutrient to keep everything in balance. Day length matters also, not just for the plants but the fish. Some plant folk and more reefers stgae their lights such that things start and end low to moderate, step up a bit and perhaps again, then step back down in mirror image of the increases. That is probably the most natural lighting program. But will it make that much difference? Personally I doubt it, but it is intellectually and aesthetically satisfying.
Your plants will do just fine with a day length between 8 and 14 hours, your fish as well. Both might do a smigen better with at least a little step-up at each end, as you avoid shock to the fish and get the plants started up at a lower rate and slowed down the same. But regularity tops the other possibilities. Using timers is the best way to operate.
If you start at x watts on for y hours, you can easily expand or contract "y" in small increments or reductions. If indivually powered you can do the same with "x" by staging start/stop times.
