My i7 2600K, built at 2011 is still going strong - OS HDD just crashed, some old fans need to be replaced. But the rest of the system is fine. Judging from the current performance meter, the system may still be usable for another 3 or more years.. unless there is a sudden change of software that eats more processing power.
 
So, I would go with the i7 (and the "K" series even if you're not overclocking for now) and a good motherboard (high grade capacitors, etc) that may see your system lasting one or two HDD life time.

I went with a cheap Dell i7 desktop with 16GB ram from Amazon.  It is a huge improvement over my previous quad core AMD.  I also have an i5 laptop with 8GB ram and the i7 runs substantially better.

You don't want to sacrifice significant clock-speed for more CPU cores.
In a perfect scenario, you want high clock-speed AND more CPU cores.
If it's a choice between significantly higher clock-speed or more cores, go with the higher clock-speed.
 
The lastest Macs are a great example:
The current top-tier Mac Pro runs a Xeon CPU (hex core).
The current top-tier iMac runs a Skylake 6700k (quad-core).
This iMac is *faster* than the current top-tier Mac Pro.
Faster clock-speed and more recent architecture...

edited.

edited.
 

"The lastest Macs are a great example:
The current top-tier Mac Pro runs a Xeon CPU (hex core).
The current top-tier iMac runs a Skylake 6700k (quad-core).
This iMac is *faster* than the current top-tier Mac Pro.
Faster clock-speed and more recent architecture... "
 
The whole iMarketing here seems a bit odd. :)

Starise
"The lastest Macs are a great example:
The current top-tier Mac Pro runs a Xeon CPU (hex core).
The current top-tier iMac runs a Skylake 6700k (quad-core).
This iMac is *faster* than the current top-tier Mac Pro.
Faster clock-speed and more recent architecture... "
 
The whole iMarketing here seems a bit odd. :)

The Mac Pro units are using Xeon CPUs... (older architecture... running at slower clock-speed).
If you're not savvy, you'd walk in and spend $4000 on the Mac Pro.
Real kick in the keister to find out the $2500 iMac is faster.
 
A PC running the same Xeon CPU/architecture would also be slower than a Skylake rig running a 6700k.
 
This technological "lag" (you take what Apple provides) is one of the major things I dislike about Mac.

Apologies for the duplicate posts. 
 
I started to wonder where this was all headed back when Apple adopted the same Intel chips as PC. 
Some Mac buyers really have no interest in seeing the numbers or knowing about the "innards". Marketing has taken the thinking out of the equation. " I want the best Mac, is that Mac Pro? I'll take it".
 
Value perception is another thing I've noticed. Some people will buy a 5 year old mac instead of a new PC. I have no problem with any of that. It means that the computers work and marketing was genius. If I were a Mac user, I believe I would at least look under the hood and compare. 
 
And I don't think people care as much if their workplace  is buying the computer for them since it isn't their money and they know that the computer will do what they want it to do.