I don’t have the 135mm DC-Nikkor–instead, I bought the 105mm DC back when I only had DX bodies. I think the 70-200mm is an excellent portrait lens, but I wanted something a bit lighter, so I bought the Sigma 150mm f/2.8 OS, which I now use for 100% of my headshots. Compared with my 105mm DC, the Sigma is sharp and contrasty, and has more neutral color rendering, whereas the 105mm DC tends to be warmer, and lower in contrast (which is part of its charm). I shot some tests a while back which showed these differences more dramatically, but couldn’t find them at the moment, so I just shot a couple of new test frames (where, unfortunately, these differences aren’t as apparent):
But it’s a tough call, since the DC lenses produce a unique image, and with the DC ring in extreme positions, can produce some really nice effects (think 1970s Penthouse layouts). As I said, the 70-200mm and Sigma 150mm are both tack-sharp (with the 150mm being easier to hand-hold). The 135mm DC should provide just enough compression for most headshots on FX bodies, and be a bit easier to hand-hold as well. In the meantime, I’ll keep looking for the other comparison shots I mentioned.
My particular copy of the 105mm DC is actually sharpest at R = f/2, and not in the detent position. It’s also worth mentioning that the DC’s effect on bokeh, per se, is often misunderstood, and/or overestimated. When setting the R-value equal to the aperture being used, its effects are actually quite subtle. And, as Derrel states, many have reported focusing issues with their DC lenses, and experiences vary by body as well. I consider the DC lenses specialty lenses. For brass-tacks, get it shot, and get it sharp, I tend to trust other lenses more. That said, it’s a unique lens with a strong following among those who manage to shoot their copies in focus.