Well, not really. This isn't about comparing the two cameras. It's a decision I made a while back to buy and use the Ricoh GR 28mm APS-C sensor camera instead of the Sigma DP1M at that time. Why? Because the Ricoh has a lot of things going for it. Both cameras have the 28mm lens, and actually the Sigma DP Merrill lens and sensor produce sharper results. The size of this camera is small enough for a pocket or jacket pocket. The Sigma isn't. The snap focus feature is awesome. The DP1M doesn't have that feature either. I already have the DP2M and DP3M, so I wanted a camera with more versatility in ISO range and features, but I do realize I give up some of the finer details produced by the Sigma DP Merrill Foveon sensor. It's a trade off. I trade off the 3D and detail Foveon sensor for the all-around smaller Ricoh camera. The lens fills the niche I needed. The Ricoh can be used at high and low ISO without losing quality in the file. It beats another camera I use, the Sony NEX 6 with kit lens. The Ricoh lacks the EVF viewfinder so, that is a definite negative. But I'm willing to give up the viewfinder on this camera. It produces lovely B&W and color images. So, it isn't as versatile as the Sony NEX 6 with viewfinder, ability to change lenses or tilt screen, but it's smaller and the lens is sharper. It's not as 3D and detailed a file output as the Sigma DP1M, but it is more useable at higher ISOs and it's smaller and the ability to set up this camera in snap focus among other features makes it a great all around camera. It's also a wonderful street camera for those who are into that.
Some things I don't like: It hunts focus in low light, no viewfinder, no ability to change lenses, some blurry shots due to NO stabilization in the camera or lens. Some blurry shots from missed focus too I think.
I'll keep shooting with this camera because it has a fun factor to it. It's easy to use, and the lens produces sharp pictures, as sharp as a Leica Summicron. Yes, it's that nice. Lloyd Chambers talks about this in his Ricoh GR reviews. Check out his website here. You will have to pay to read it, but there is so much information about all of these brands and worthy of a look.
I think this camera is great for street photography, travel, and everyday use. It is limited in its focal length. That could be good for some, bad for others.