The one thing that sets tdm or pt hd apart is:
very low latency,
If you are multitracking full bands, this can be an important issue.
I agree with most of the rest of the posts here.
PT hd is the standard now, If you are doing a lot of work with other studios it can make a difference in lost time converting stripes to use in a pc format nuendo rig.
That being said, if you do most of your work in house, and maybe not huge tracking sessions you can be just as happy with a pc nuendo rig for much less $$. You can get very low latency now, esp if only recording a couple tracks at a time.
48K A/B of my pt le system with 32bit float in nuendo?...(with the same dac and adc,)
Nuendo wins, I havent been able to a/b nuendo and hd yet, but I have a feeling the steinberg rig would win.
If you are a composer/writer type I think the VSTi and midi tools in nuendo are better than pt, something to consider.