Buy if You're Going to Live With it For a Long Time
Do you plan to buy the 2018 MacBook Pro and keep it for several years? If so, it's always a good idea to buy the latest model and get enough new technology and performance to last.
Not everyone needs the best processor or 32GB of RAM, but even if you opt for a base or mid-line 2018 MacBook Pro, it's going to last you longer and serve you better than buying an older model if you keep your notebook until it cannot do the work you need anymore.
For users who think they will upgrade in a year or two, it may be a better option to look for a killer deal on an older MacBook Pro model during closeouts or from someone who is upgrading.
I typically keep my MacBook Pro for several years, so I'm not due for an upgrade yet. If I'm lucky, my 2016 MacBook Pro will last me through at least another update or two before I need to buy one.