Thanks, folks.
The reason why buying an ECC system is not because non-ECC may crash, but is it may silently corrupt my data, especially I do sophisticated board level design, and occasionally do simple ASIC design.
I'd prefer to build a high performance and solid machine, because I want it to last longer. The cost is not really that important, because an AD15 will cost me 9k+ USD (I'm currently using academic AD14, but I'll graduate in 1 year, and I want to stay in USA to find a job).
Since major version upgrade of AD is VERY expensive, so I will not upgrade to A16+. I'll stick with AD15, so I really don't need it to be super future proof.
Here my question is whether I should go for hex core quad channel DDR4, or I can go for quad core dual channel DDR3. Both is good, and both can handle AD15 perfectly. GPU may be upgraded later, but that is completely for gaming. I don't do 3D modeling using any professional tools, and I even don't use AD's 3D PCB function.
I MUST go with a SFF case, because I'm living in a very space confined place, and I have 2 monitors and another intel NUC server in my dorm. A standard ATX just won't fit in my room. Personally I will go for quad core+dual channel DDR3, because it is really energy efficient, but my concern is I may encounter huge trouble upgrading the system later, if I need better performance. If I go for LGA1150, then I can't upgrade it later, I'll have to buy a new one, since this is probably the best LGA1150 platform ever made. If I go with LGA2011, then I have quite a margin to tinkering my system.
Another question for you guys is DO I REALLY NEED QUAD CHANNEL DDR4? It has 3x bw compared to 2*DDR3 1600, and is not so expensive. Will I encounter memory performance barrier in everyday using?