Your question is rather confusing! There is no such current software as Movie Edit Pro 16 and Movie Edit Pro Plus 16. Movie Edit Pro (current version 19) exists in three variants, Basic, Plus and Premium. For a comparison of these variants please see this web page. If you are considering purchasing the software the advice here is always to purchase Premium since it offers the greatest number of facilities, and is currently actually cheaper than Plus.
Magix Vegas is one of the video products acquired when Sony sold most of its video and audio products to Magix several years ago. There is a dedicated Vegas forum here.
I agree it is confusing. I did misspeak and apologize. I have Movie Edit Pro Plus 18. So what is the "Plus"? So the next upgrade to this is Magix Movie Edit Pro Premium. What is the difference then between Pro, Plus, Premium?
Upgrades are usually to the next version of the same level, from Plus to Plus, Premium to Premium for version 2019 to 2020. You actually have MEP 2019 Plus which is version 18.xxxx when you look at the version code.
The latest is MEP 2020 which is what you want to upgrade to. If you want change from Plus to Premium, then that is slightly different, but still from 2019 to 2020.
There is no Pro version. There is Movie Edit Pro (basic, Plus & Premium) and Video Pro X. Here is the link to the comparison table between MEP Premium and Video Pro X.
All of this information and more is on the Magix.com web site and you should review the details.
Vegas is a separate program and there is no comparison table. Thus, you should look at what MEP offers and what Vegas offers in its different flavours by reviewing the information on the web site by selecting Vegas from the menu.