as said previously straight 6's arent true muscle. If muscle is your desire go for the V8. Your already in a real performance car with the STi so the performance factor shouldnt really sway you.
Also a good point was made about the GTS and the Clubby. You could save some money and do without the 20s, the badge, the MCS(?) (magnetic suspension thing -a- me), but I suspect the reason you want muscle is for its lazy, effortless torque which makes for a nice cruiser? ( of course I am just assuming here) If that is the case why not look at a Senator? They depreciate like a biatch which means you could score a great deal. They are super plush and comfy, same V8 and the boot lip spoiler is uber cool!
As always its your money and your decision, take em all for a drive and see which one fits in with your reason for purchasing in the first place.

As a side note, in my experience with detailing many examples of both FPV and HSV, the Fords seem to fall apart quicker. things like rust coming through in the boot drainage channels, and the same in the engine bay. interior trim coming loose, marking easily etc etc. Also the Ford paint is softer than the Holden and can mar up quicker, but since youre already in a Subaru (with epically soft paint) that shouldnt really matter

Good luck with the decision.