Hi and welcome!
I guess you set the poll by accident. It is really not a matter of opinion.
You can do it either way. I'd recommend to keep MJ12 installed (I assume you mean MJ12 by MJ2008) until you have become familiar with MC13 and are ready to switch completely.
If you have done lots of work with your MJ12 library you could create a new library backup file before proceeding (just as a precaution). Store the created backup zip file in a safe place, preferably on a separate disk. (File > Library > Back Up Library...)
You can use the library restore tool for importing the backup file to MC13 in case MC13 didn't automatically find and import the MJ12 library.
Before unistalling MJ12 you should check the cover art location. The default location is under the application data folder and if you have stored cover art files there the cover art files will be deleted if the unistaller is used with all cleaning options set.
MC13 has a tool that can copy linked cover art to a new location (as set in the file location options) and relink the images automatically.
EDIT
StFeder and JimH posted while I was writing my reply. (I was interrupted before the reply was finished.)
As said, if the old version really is MJ8 the situation is different. However, I think MC13 can read an MJ8 library backup file when it is imported by using the restore tool. (Some time ago the MJ8 library reader was returned to MC -- if it is removed again you can use MC12 as an intermediate version.)
If you have MJ8, please post more details about your setup and explain what you would like to preserve.