It's fight between power and portability. If you want more power than your current MBP offers, get iMac. If you want to take your Mac to a friend or a vacation don't do anything, MBP is portable while iMac isn't. It's your decicion.
I would say if you dont take your computer everywhere than take the imac. I have an iMac and I have no problem moving it around my house and it would be more powerful (but its three years old 🙁)
Yes, if you really want to have the new iMac and you can live without the portability. The MacBook Pro has a great resale value, especially one that new if it is in good condition. After resale, you would probably only spend a couple hundred dollars, or possibly none at all, for the new iMac. In the mean time you have a great machine to use.