Keep the spare ball the same weight or a fraction lighter, unless you are used to changing up and down in weight a lot. The nice thing about a spare ball is that it is probably the only piece of equipment that you will always throw almost every game.
Going a bit lighter has the advantage that you can throw it a bit harder on really dry lanes if you need to, and gives you a little more leeway on spares like the 3-10. I used a 14lb spare ball and 15/16 lb strike ball for a long time. More than a pound lighter can lead to problems with the swing, especially for power players. If you have a tight thumb, you might tend to hang up a bit more in a much lighter ball, too.