This topic has been covered in many places, many times. Just my opinion here (distilled down from years of research but also first-hand experience with some nice hardware on both sides of the argument, mind you), but I'm fairly certain the apex IRS build will outperform the apex SRA build... when it comes to street/highway/circuit/autocross/road racing. The SRA will ALWAYS be king at the drags.
It's not to say the SRA can't hang in the twisties... far from it! As pictured, I had a Fays2 watts link and an upper third link from Evolution Motorsport (extinct) that was loads of fun in the mountains. Dare I say almost as good as the current IRS I have. With the right gear, the right alignment specs, the know-how, etc.., the SRA can come very close to the IRS's level of handling performance. But like I said the fully built IRS will always have that slight edge over the fully built SRA.
And that's where the real value proposition comes in for the SRA. With minimal money (compared to getting the IRS to where it's ideally at), the SRA will match it in performance except in the realm of comfort.
You will spend more money beefing up an IRS--generally-- than you will beefing up an SRA. Unless of course you're talking about a Cortex Racing cambered drive stick axle, then I really can't say anything but bravo well done.
The IRS is definitely not overbuilt from Ford. Those rear cantilever mounts, while strong, DO flex under extreme application. That's why people like Kenny Brown and even individual enthusiasts are chopping them off and welding up brackets that bolt directly to the frame rails. The forward diff torque brace, while beefy, is not that great. To get wheel hop down to a minimum, mounts other than the factory are required AT MINIMUM. The factory IRS aluminum control arms absolutely flex in extreme conditions, and so do the uprights for that matter, putting stress on the weak ass bearing. These things are all documented by people like Kenny Brown over the years of actual racing.
Street? No biggy.
See my pics over the years...