I agree too early to tell if roo or pullet at this point. Usually, the roo will get a nice red, fleshy comb earlier...but I agree Wyandottes can leave you guessing for some.
As to which to keep as a roo (assuming you have an SLW roo as well as the Cuckoo Marans roos), keep the best nature rooster. Never keep a bird that is snotty to you as a chick as they grow up snotty (and make nice chicken and dumplings). Also always keep the rooster that is best for type...no flaws (cross beak, crooked tail, poor build, unthrifty as a chick).
If all that is equal, as to Marans or Wyandotte...depends upon what you want to do. In my experience, Marans and Wyandottes are pretty similar in temperament and type.
Neither are super docile, but neither overly aggressive. I would call both breeds assertive birds but generally calm and not overly noisy. Important to consider when breeding forward or keeping a rooster.
Both breeds have been similar layers for me...decent to good, but not prolific. Both are heavier birds so provide a decent carcass (if that matters to you). So as to general types, they are pretty equal.
Assuming then all equal as to health and temperament and general type ....you might further consider...
If you keep and breed the Marans, you may improve egg color for the Marans, selecting for the darker layers over time, and may even darken the whole flock over time with some darker wash...usually if one parent is darker brown and the other lighter, the resulting egg shell color is mid-way. So if you've got darker layers in the Marans, a darker roo will eventually darken all the eggs (theoretically).
If you have blue egg layers on the farm, a darker layer roo will create olive eggers (and from my research, the dark brown roo over the blue gene hen produces the darker olive vs. blue gene roo over dark brown layer hen).
However, cuckoo on the rooster side will bar everything, so all chicks will come out cuckoo, both sexes, no matter what you put him over. In time you will have cuckoo/barred birds for the whole flock (assuming you keep breeding back).
Now if you keep the SLW roo, placing him over your Cuckoo Marans girls (and any other barred hen) will give you sexlinks...boys will be barred (white head dot) and girls will be black...a really nice feature.
As stated earlier, both Wyandottes and Marans are considered dual purpose, so you should get decent egg layers and a decent carcass weight, if you want to go that direction. Having sex linking adds another advantage of the ability to step up protein for the boys to finish them out for the pot faster (though of course not as fast as a true meat bird).
Another advantage will be that the SLW is rose comb, which is dominant, and your subsequent chicks will carry the rose comb forward (mostly...there is some single comb throw backs in Wyandottes so it depends upon your SLW boy's genetics, assuming this turns out to be a boy, if he has only a single rose comb gene). If cold winters are an issue, having the rose comb vs. the larger single Marans comb could be advantageous.
So it really depends on what you want.
Edited by Lady of McCamley - 3/21/16 at 12:00am