Two very different pitchers.
Marshall, a soft tosser from the left side with a lot of breaking stuff.
Samardzija, a hard thrower from the right side.
The Cubs have excess bullpen pitchers right now and will continue to for next year if all goes according to plan. Zambrano might be on the way out but even if not him and Dempster will both be gone after next year unless extended, and the Cubs might need a replacement.
Marshall is the better pitcher over the last many years, but Samardzija has come on strong lately. Normally the best pitchers are starters but it doesn't always work that way.
Looking at the numbers, it's obvious who deserves the rotation spot.
Jeff Samardzija, though he is not without his weaknesses. He has a huge split, he's pretty bad vs lefties, walking one per inning. I don't think he can possibly stay that bad, and if he can have it at 4-5 over a full season as a start it would be tolerable as long as he was better against righties.
Marshall throws way too many slides and curveballs to make a transition to the rotation, where he would have to depend heavily on his fastball. That is a weakness for him, and a strength for Samardzija.
Samardzija's other weakness is the walk rate. Overall at 5.37 walks/9 innings. This his improved considerably since the beginning of the year and might be below 4 going forward.
His repetoir consists of a blazing fastball, hard slider, and split finger. Against lefties he might need to lean away from the slider a bit, but he's still got 2 other pitches. Adding another, like a cutter would help considerably if he could pull it off. He doesn't appear to have ace potential but can still get the job done.
All I am saying is give Samardzija a chance. And give peace a chance too.