I used to use MacBook Pros. Eventually I realized that my MacBook Pros were practically glued to my desk, so when it came time to upgrade, I went with an iMac, and bought a MacBook for a portable. The MacBook isn't powerful enough to be my primary computer, but it's more mobile than my old MacBook Pro was.
The advice, as always, depends on your use cases. Two laptops (a MacBook and MacBook Pro) seems redundant; is your MacBook Pro largely stationary? If it is, the iMac might be a consideration. But that's really it; I haven't seen recent benchmarks to say for certain, but the MacBook Pro usually isn't that far behind the iMacs in terms of performance (and a higher-end MacBook Pro can beat a lower-end iMac). If you like the monitor, you can consider getting a high-resolution monitor and plugging it into your MacBook Pro to get about the same effect.
I don't think it makes sense to have all three systems, though. If you get the iMac, get rid of the MacBook Pro (unless you do mobile video editing, or something requiring the portable power). If the MacBook Pro is working fine for you, consider passing on the iMac until your MacBook Pro is in need of replacing.