FWIW, I've been testing Sam8 and Nuendo 3.2 on the same X2 4400 dual core system to compare workflow and performance mainly. You might be surprised at the results. Short answer: I would download the Sam 8.11 demo and try it yourself. It doesn't have the rich feature set Cubase SX does (or Nuendo), or the nice GUI, but it makes up for some of that in other areas:
1) based on what I am seeing in test comparisons, MUCH better low latency performance (twice the performance as Nuendo on this system at 1.5ms latency when comparing plugin counts and large sample library-VSTi counts - separate tests);
2) object oriented effects
3) CD burning/mastering in the same app.
4) Very nice built in effects
I'll post more info after I confirm a few things with developers and other users. FWIW, my Nuendo rig has performed identically to other X2 4400 rigs in other performance tests, so my system is stable, and efficient. I am not seeing any issues with stability in Samplitude here. It's stable and very fast. I even got it to peg the cpu at 548% load. :lol: That is of no use other than understanding how it handles extreme loads. Yes, it was distorted, but Sam seems to start dropping out parts of the audio stream, softening the chacophony rather than momentarily freezing the system, which is nice - it seems smoother in handling large loads, and almost unaffected - allowing the user to back off plugin counts, adjust VIP buffers, etc. to compensate without disrupting workflow. When you are trying to get the most out of your rig, that is important.
I work in Nuendo everyday, love what I can do with it, and know it's limits, weak points and strong points quite well now. I'm just looking at the differences to determine which best suits each part of my workflow requirements, so that's my perspective in posting this.