You said they have similar IQ for outdoor- is there a big difference when shooting indoor with studio lighting?
I have two studio lights(about 1000W each). When I used them in my loft(about 16x16), I still can have ISO 200. The IQ for both 7D & 5D2 are similar. 5D2 might be a little bit better, but not much. However, I don't use studio lights now because the photos with studio lighting are not natural for me. Without good light source, you will see the benefit from FF(my 5D2).
Historically, I've held back from shooting anything above 400 ISO. I know ISO performance has improved dramatically since the XTi. I've only used the 5D Mark II a few times, but I did notice a big difference. And the Mark III seems to be an even bigger difference.
XTi uses very old sensor, ISO 400 may not as good as ISO 1600 or even 3200 on 5D2. Also 5D2 has much more MP, that will make you feel better IQ.
The 5D3 may have only one stop better IQ than 5D2 in RAW file. However, 5D3 has much better AF system.
If you have to ask that question, no it doesn't worth the price.
It does worth the price to you if you don't need to ask this because you already know what it can do.
Agree the second one, but not the first one. That why shunsai comes here to get ideas from us.
Everyone has their own value for performance, features, and IQ. All of them can be traded off. Thus, don't believe anyone when they go with 5D3 or go with T4i. That's their choice, not yours. You only need to know what's good on 5D3 and T4i, and make your decisions.