pigswill said:
If you get the same production by running HE and Ironworks in seperate cities as one city then it comes down to personal style and preference.
it's not just personnal style
there are some pure facts :
Fact 1:
Heroic epic comes early (most of the time), and you want to build units there almost non-stop.
Each turn where you build anything else is a turn where you lose the 100% bonus.
Fact 2:
You need more than 1 good production city. Be it for building wonders, SS parts, or more units.
If your HE city is your only good production city, you'll have a really hard time building anything, including the RC, WP, Globe theater, ...
Fact 3:
there are 3 different national wonders affecting military. 1 about speed (HE), 2 about quality (WP, RC).
Quality without quantity is worth nothing (not true the other way round : quantity without quality can be enough).
So if you want to put the quality things to a good use, you need to pair them with high production.
WP is obviously more flexible than RC, WP city needs ti churn out as many units as possible. RC city can also be used for other things (wonders, SS parts), but you can use a few marching (medic + 1 promotion!) units

.
That gives an optimum configuration, where HE and WP are paired (barrack is needed too, of course), and RC is simply paired with a barrack and some good production.
Since you want to put IW in a good production city, and the major unit city already has all national wonders built, the RC city is a good candidate.
Not the only one though.
I'm going to try for a national epic / IW combination, with loads of GE
