I agree with what's been said, it's what feels right to you. My preference for length is 7' or longer, but I also have 6'6" rods that I like to use. Everyone has there preferences.
Many rods now are made for specific applications, but that doesn't mean they can't be used for other techniques. As far as general purpose, that may depend a lot on what techniques you'll use them for, as either rod might work well, or neither. Just as an example, I like flipping jigs with a longer rod, 7 1/2' long, and even have one rod, which was marketed as a Steelhead/Salmon jig rod, that's 8'6" long, and works great for flipping light jigs. For me, a 7' might work too, but likely not as well, for me.
I feel that both those lengths are good for casting, like for spinnerbaits, crankbaits, jerkbaits, etc, but may not be ideal for other purposes. That's something that you'll need to figure out, and much of that will be based on how they feel to you.