Best Performance for Thermals: CPUs with less cores or under-clocked CPUs

[TheAsianHan]

As the title states, I am curious which will yield better performance in a less than ideal environment: a CPU with less cores like the Intel Pentium G3258 (Anniversary Edition) with possible overclock if thermals allow it or a under-clocked CPU such as the Intel Core i7 4790k running at a slower clock speed. The reason I am curious about this is because I want to build a small form factor gaming rig, something that could fit in the ASRock M8 case for example. The only parameter is that there is no room for any water cooling solutions, just air cooled solutions. 

[qwertywarrior]

with air cooling u can run a 4790k just fine

[Peter Vaughan Truslow]

get a quad core and run it with whatever low profile cooler you can. even the stock intel cooler is okay at stock speeds, but it isn't as quiet as an aftermarket cooler.

[Manny Calavera]

underclocked or no turbo and undervolted xeon 1231v3

[TheAsianHan]

Thanks for all the responses guys!