Do you understand that you have an opportunity to see what each service offers before you pay for it? If you want to watch the World Series, take some time to research a few days or a couple weeks before the series starts and figure out which service you need. It's not like it's being sprung on you at the last minute.
On Roku, the Series can be watched on FOX Sports Go----But you need a cable or satellite subscription to log in
If you have a Sling TV subscription that includes Fox you can use it to log into Fox Sports Go. It is just like a cable subscription.
Let;s see, the Super Bowl is 4 months away. Better start doing research now to see if it will be on streaming media and where.
There's supposedly HULU TV and Direct TV NOW and some others that have the games.... But naaaw. I give up trying to watch the world series.