Sproul was our best defender for a while there, playing great hockey, while paired with Paetsch. The past few games he has been paired with Brennan Evans and he hasn't looked near as good. A lot can be said about the veteran presence of a guy like Nathan Paetsch, he makes whoever he is paired with, that much better. There's no question that Marchenko is our most consistent, and definitely most NHL ready defenseman. He is more of a shut down defender but can put up some points as well. He doesn't shoot very often, despite having a pretty decent shot, he is also a very underrated passer. He leads the Griffins defense in points, despite getting very little power-play time, spending most of his time at even strength and penalty kill.
Mantha hasn't been playing as well as many had anticipated, but he's right where I figured he would be at this point. I said that I think he will start to pick it up after Christmas and I still expect that to be the case. He has been playing much better as of late, although he did have a pretty forgettable game last night, with a couple horrendous turnovers. The thing that I still see Mantha lacking, is what he was labeled for in his draft year, up until he supposedly proved everyone wrong in his final year of junior. He lacks compete level. Often times he will give the puck away and then show little to no effort in trying to get it back. He doesn't back check the way he should, and until he improves his defensive game, I don't see him making the jump to the NHL. I'm sure Blashill has had talks with him and I'm sure he will get it straightened out, but until then, he will remain down in Grand Rapids.
I agree with your assessment of Marchenko. He's a "shutdown defender" in the way that Dekeyser is. Which is to say that he's not known for his offense, and he definitely prioritizes his defense and plays a safe game. But he's also an effective puck mover and is really good at getting the puck up the ice. I actually think he'll put up more points at the NHL level than he does in the AHL when he's surrounded by more first rate finishers.
Edit: As far as Mantha goes, I think he struggles from the same thing that Jurco does. He's a big guy, and at the pro level will be expected to play a big man's game. But until he came to GR, he played a skilled, finesse game, despite his size. He's basically learning how to be a power forward at the pro level and the learning curve is steep. Once he figures out the power forward's game (i.e. where to be on the ice, how to play without the puck, etc.) he'll be fine. But until now he's always played the way a center has to play at the pro level (carry the puck and make plays on individual effort).