David Helton

When you say "worth" - what do you mean?  Because I used to fund my comic habit almost exclusively by buying moderns raw in the $1-50 range and then slabbing them and flipping them for $100+ - sometimes blue label sometimes yellow label. If you can grade, and you know what is hot, there are dozens and dozens and dozens of books from the last 30 years that are worth slabbing.

Here are some things to consider...
Do you prefer the look of the bag or the cover of the Superman 75 platinum edition? Google if you have never seen...if you prefer one over the other, that could help your decision.
As for value, yes your book is sealed but that does not guarantee a 9.8 copy. Most likely it has succumbed to some wear in is life and might be a 9.4-9.6 copy. Graded 9.4 copies sell for around $120-150. 9.6's, $200-225. If you keep it in the bag, sales seem consistent around $150-200 range. So if this is purely a money move (or "investment" move, consider this).
Finally, the integrity of the comic. While true that the plastic could have adverse effects on the book, the plastics from the 70's (and comics from the 70's) are very different from plastics in the 90's (and comics from the 90's). A 90's comic in a bag and stored in an environment without extreme temp changes or humidity should last longer than you lifetime. 
What might be the winning answer is getting the comic graded and keeping the bag in its own bag and board. You know, for nostalgia purposes. That way when selling (if that is the goal) you have a graded copy that is hopefully a high grade (only way to get an idea is to open the bag...could be a 9.6 but could have sustained damage and be am 8.5) and you also have the original bag as an interest piece to go with it.
 

So I have a Superman 75 platinum edition Still in polybag never opened But I've heard several different Things some people say leave it in bag and others say have it slabed what should I do and if I do decide to have it slabed how should I go about it