You might want to consider none of the above
I'm in the same situation (sort of). I am choosing between the K5000 for $1800 or a used quadro 6000 for the same price or less. I'm also slightly better looking than you too
otherwise, same situation.
I'm building a workstation as part of a client solution and I'm finding the literature on the new cards to be somewhere between confusing and misleading.
The official blurb implies that I need to add a Tesla to get good rendering performance with the K5000, taking the cost to north of $5000 which causes my butt-hole to pucker a little.
If I chose the quadro 6000, viewport will be slower but it will give me solid rendering performance. Sounds like a huge step backwards to me. I want once card, not 2 but I also want my card to be faster than anyone elses. You just don’t give flowers without a vase!
General view on AMD seems to be that they are either a bunch of useless morons on the brink of bankruptcy with no driver support or geniuses who sell superior cards to Nvidia for lower costs (although he latter is not true for pro cards). I don't know but suspect HP don't offer them as options because demand is lower but that shouldn't matter to you if your apps have optimized drivers. There are plenty of AMD benchmarks for all of your programs.
You are meant to chose the card that works for your software. I have seen benchmarks showing that the AMD cards are many times faster for Maya and the reverse for 3DS. I never found that to be true for Maya but that doesn't mean anything. Maya and 3DS do use different engines (Open GL vs Direct X) so it might be time to narrow it down. General view seems to be that Nvidia cards are more widely used and therefore have more time spent on optimizing drivers for the apps that most people (including you) use.
I know this is no help but if anyone with hands on experience could give us both a view on the K5000 for Maya, I'd really appreciate it. Oh, and same for the Quadro 6000 for Maya 2013. At least this one is for a client so I won't have to live with the shame of making the wrong choice. Just close your eyes and pick one. They are easy to sell on Ebay if it doesn't work!
If only here was somewhere I could go to try them out before I buy...