it doesn't hurt to have both. but day in day out I catch most of my fish on harnesses. I believe most guys are going to say harnesses but im sure there is a few guys that will swear by spoons. and I do mix a spoon or two in every once in awhile. the only thing is you really need to run a little faster with spoons. in the summer months I like to run about 1.8 to 2.0 so I can get away with mixing them up a little. but if you run all spoons you can run a little faster. this is just my opinion on the harnesses. but if I had to pick just one it would be the harness.
I second Sherman51 on the harnesses versus spoons. I have both but seldom use the spoons.
On the harnesses the conventional wisdom is colder water use colorado blades at slower speeds (1 to 1.5 mph) as the water warms up pick up the speed to closer to 2 mph and move to willow blades or double willows.
I do have to confess for the most part I stick to the slower speeds with colorados even as the water warms up but the last 3 times I have been out I have gotten my limit on double willows at 2 mph.
I still use spoons may/june/july, spoons and jets. Limits of walleye. I must be old school, but it works out as the big old walleye are old school also. Screw the worms, dirt, junk fish as well as all the BS that goes with harnesses.