If your serious about the 500whp, no I mean serious
... don't waste your time or money on either one. Go stright to a stand alone. Here's why;
I've only done one S-AFC tune on top of a flash and it was the "custom road tune" too. It was conservative and the owner/buddy of mine asked me to see what I could do. I said OK and I learned one thing... IT'S A PAIN IN THE A$$.
After numerous runs and drastic changes in the S-AFC settings, in both directions, I could not get the timing to budge. After some more seaching a questioning the owner and I settled on the idea that the programmed timing is locked. This makes road tuning with a logger very dangerous because the logger detects knock through timing. It might be OK to take the flash a little farther on a dyno, with a wideband and a seperate knock device.
So if you go this route, add the costs; The flash its self, his road tune, the cost of an S-AFC, dyno time (+wideband), seperate knock device (to really do it right)... That's the cost of AEM's EMS, which you know will give you all the control and support for more than 500whp.
On top of that, what size injectors are going to be needed to support 500whp? Are you sure a flash+S-AFC can control it?