You have a lot of options as there is a wide gap between those two balls.
If you want another solid the VR solid would be a good choice and a step down in hook potential from the Nano.
A hybrid ball may be your best option as it will fit the best between these two balls. You can't go wrong with the Hyroad. The Reign Supreme is another good hybrid ball that for me is smoother than the hyroad. I have not thrown the Infinite Theory so I can't comment on that one.
Another option you may want to consider is the Marvel Pearl. It would also fit good between these two balls. It really depends on what kind of ball motion you are looking for in the middle. The VR solid is going to be the earliest and smoothest. The hybrid balls will be in between and the Marvel Pearl will be more skid/flippy but more aggressive than the victory rd.
