The K Series s the newest generation card. I would get the K5000.
I am sure you have investigated the choice for hardware, but often a Z420 with higher clocked quadcore CPU (3,6 GHz to be exact) is best for SolidWorks users. I don't know about the other software you use, but there isn't much software available that will take full advantage of the multi CPU setup. 90 % of the SolidWorks users I know who stretched their budget to a Z620 or Z820 were disapointed.
If you use Simulation or photoview360 a lot (daily) it could be a good choice. Be sure to spend well over $ 3000 on this system (without the K5000 and only one CPU@3GHz+) to be just short of the Z420 in performance for SolidWorks.
IMHO it's only worth it if you plan and need a second processor in the next 1 to 1,5 years or really need the memory that you can put into the Z820 (512GB compared to 64GB for the Z420).
To answer you're initial question I would go for the K5000 as well.
Thank you very much for everybody's advice. I shall get the K5000 then.