8GB RAM and IPS screen vs 16GB RAM and TN screen?

A quality monitor is the best possible investment for a computer. Chances are you'll keep it far longer than any other piece of equipment, especially if you really go all out and get something 30" plus.
 
Once you go IPS, you never go back. Once you go 16GB, you never even notice if the other system has only 8GB (unless you do heavy rendering). + you can add more RAM later, but you'd need to sell the TN panel when upgrading to IPS.

Go for the better monitor.
 
I should expand on that... not only will usually you keep your monitor longer than any other component, it's the thing that you actually look at while you're computing... it shows you what your computer is doing, and is your workspace, etc.

At the same time, your monitor really sets the requirements for your PC build. If you've got a 4k monitor, you'll need a system that's 4x as powerful as you would if you were using a 2k monitor. Same with refresh rates — if you're doing a 120+ hz gaming monitor, you'll need double the graphics card muscle to reach a framerate that takes advantage of that.

It always makes me facepalm when I see someone build a fancy $1200+ gaming PC then put a $120 23" monitor on it.