I thought you didnt like SABR stats?
In terms of WAR it's not going to be that much because they don't pitch that many innings.
Going off just last year which is really the only sample size we have its about a win difference in terms of WAR.
Now I am going to get all technical on you....
The reason its even that large is because of the LI (leverage index) (which measures how "pressurized" the appearance is. Basically the higher the LI the more important the situation is. For instance pitching in the first inning tie score is going to be around a 1 where as pitching in the 9th inning bases loaded up by 1 is going to be around an 8ish or higher. Like for instance our game the other night with the Astros where Marshall was pitching to Bourn with the bases loaded top of the 7th in a tie game the LI was 4.73
here you can see all the LI for that game but again this was just a quick lesson). If you would like me to say more I would be happy to.
Now that I got off track the reason the LI matters is because its weighted differently. A relief pitcher that pitches like Marmol does in high leverage situation will be and should be worth more than someone who pitches like Marmol in only mop up duty, and since Marmol's LI is higher than Marshall's he will have the higher WAR.
If you want to just look at stats like ERA or FIP the difference isn't as large. Over 70-ish innings is somewhere between 1 and 2 runs a year which is no where near a win share. Just depends on whether you think Marshall can compete like he currently does as the closer.
I still wouldn't move Marmol just because we now have the 8th and 9th innings basically locked down. That's something not a lot of teams have and this team isn't nearly as far off as all of you think it is. We could VERY VERY easily be looking at a 2006 type team a team riddled by injuries with some money coming off and a very nice FA in spots we need players. We have 5 very solid SP's a lock down back end of the bullpen and just a few weaknesses on the offense that can easily be corrected.
Marmol should not even be considered as a trade unless you are getting a mammoth return, and by that I am talking someone who is going to make an impact immediately.