If the car is to be used primarily (read drags) for straight line driving go with the solid rear end. If it's going to be a road car I'd recommend the IRS. The thing to keep in mind with the IRS is that for the occasional hard launch you might want to triangulate the rear end with a couple of additional radius arms to prevent wheel hop. There have been a couple of threads addressing wheel hop with the IRS. Not a particular problem to correct.
I've got the modified IRS in my ERA with the outboard disks and have never had a wheel hop problem with it on hard launches. The setup has two radius arms (top and bottom of the hub carrier). Here's a link to a quick look page to see the type of setup they use.
http://www.erareplicas.com/427/frsusp.htm
DonC