Congrats on the turn around
In this case, I think you are better off keeping what you have. Burrell is fast becoming a really good hitter, not just for power either.
No way you make this trade if it's a keeper league.
El Duque may just get a couple showcase starts as the Yankees are trying to deal him. I don't think he has much on Marquis if he's traded away as his win totals will surely go down and I don't like him if he stays because Weaver will get the starts and El Duque will be in the pen.
One Thought: Now that you're in the playoff race, here's what I can see as a possible plus to getting Edmonds. The fantasy playoffs happen the last couple weeks of the season, those games "MIGHT BE" important to STL, meaning Edmonds plays. (I really think the Cards will run away after they get over DK and they'll rest their vets, though) No way those games matter to the Phil's so they may be giving alot of playing time to September call-ups. Just something to consider.