The 13" rMBP will most likely come with the 28W TDP Haswells. These include the newest HD5100 graphics core (Iris). Consensus seems to be that these will be significantly more capable than HD4000. They have twice as many execution units, and are a few Mhz faster. I would reccomend looking at the MacBook Air gaming reviews and expecting a decent improvement from that (some people are saying around 10-20% increase in performance). This is due to the HD5100 being clocked 100Mhz higher, and having a higher TDP to play with.
I am still holding hope that the 15" rMBP will have a dGPU but the overwhelming majority are expecting it to only come with Iris Pro HD5200 which is the same as HD5100 but with 128MB eDRAM cache to speed things up.
I will say that HD5100 is more than enough for the 13" rMPB. It will be the perfect machine if you ask me.
http://www.anandtech.com/show/6926/intel-iris-iris-pro-graphics-haswell-gt3gt3e-gets-a-brand
http://www.anandtech.com/show/7072/intel-hd-5000-vs-hd-4000-vs-hd-4400
This exactly ^
I think you personally would be almost perfectly happy with the new 13" retina assuming it is announced with the 5100. We're finally getting to a point where the lower spec'd options are actually turning out half decent (good for your type of customer). Unfortunately on the other end; for people like me, who find a 13" screen a bit too small, or want to play games on higher than medium graphics, we're stuck with a similarly spec'd 5200 in the 15" version. With the absence of a dGPU, that is. I'm not a power user, I just like games, so that's my personal suffering. But if I were actually a power user, I'd be getting shot in the foot, basically forced to digress all portability and get an iMac.
That is of course, unless you let apple tell you that this is a worthy upgrade, and better than the previous generation in every way.