I own a fair bit from Altieri and Poell.
As Leviathan said, Poell's is a more tailored line while C Diem is supposed to be ultra-casual.
If you are going for a suit or long-coat, Poell is without a doubt the guy. I tend to find his things easier to wear. They are masterfully cut clothes in beautiful fabrics with a few interesting design twists. C Diem is a whole look unto itself, not much goes with it other than more C Diem and its not really fit for formal occasions or anything requiring collared shirt, formal trousers and tie.
C Diem is actually based on work-wear, and in my opinion develops every season to get better (altieri is an artist..so his original experiments have evolved into the items we see every season in this brand today)
You can think about looking into Linea and Fattoamano, two experimental lines of Maurizio Altieri. Linea has been discontinued and is selling at maxfield, very little at A, Chegini, and Lift Etage. It runs $800-$1400 for non-leather pants, $500 for shirts, $2400+ for leather pants and shirts, and $3-5k for leather jackets. All the items are made in extreme small quantities with experimental fabrics.. they can almost be seen as one offs. The look is completely different.. being ultra-slim (more so than dior homme) with black,white,and grey as the main colors. Fattoamano is like the reborn line of Linea.. so far no us stockists for it.
However, I think if your only seeing the stock at A of these brands.. your opinion would be pretty biased because the buyers there buy very little of these two designers (everything must be slim and black.. no leather) compared to places like Lift Etage in Tokyo and Leclaireur.
Its quite hard to mix the two, with the exception of the boots.