Looking at the DS918+ is it worth waiting for the DS920+ or is not that much of a difference
If you plan on using virtual machines DS920+ will be a better choice since CPU and RAM is faster.
But if you need it for multimedia DS918+ will do a better job. DS920+ has worse support for 4K and h265 encoding.
There will be no difference in performance on other things like file server and data transfer speeds and so on.