I can't say what you'd regret. Me, I got a cMBP.
Reasons:
1. RAM and HDD upgrades.
2. Ethernet and firewire ports. I still use these both actively. I am not okay with needing dongles for them, because if I have dongles for both of those, I am out of video ports. (HDMI will not drive a 2560x1600 display...)
3. Antiglare display. I can't stand glossy. Yes, I've seen the "less" reflective Retina display. It's still too glossy for me, and I can't use it.
I would say, give serious consideration to going for 16GB of memory. It won't cost that much more, and then you don't end up buying it anyway a year later and having 8GB of memory you have no use for.
You don't mention the size of SSD, or your usual use environment, so I don't know how much it'll matter to you whether you have access to more space easily. SSD prices drop rapidly, so don't feel too pressured to get the absolute largest you can. And of course, if you get the classic, you can use a firewire 800 drive with it.
The native HDMI port is indeed a nice thing; one of the nice things about the 2012 mini is having thunderbolt open while using an HDMI display. But overall, I've been pretty happy with the basic premises of the cMBP, and the Retina display does not suit my needs as well; at resolutions other than 2880x1800, it looks worse to me than a plain 1920x1200 or 1680x1050 display, it's glossy, and so on. In practice, it's a 1440x900 display with really sharp pixels, and while it really is very pretty, that's not enough screen real estate; increasing the effective real estate really hurts the clarity, MHO.
But me... I'd regret the rMBP, because of the glossy display and total lack of future upgrade options. It comes down to personal taste and likely use cases. I tend to use machines for the full three years of their AppleCare, and by three years from now, I am quite confident I will have replaced the 480GB SSD I put in my machine with a larger one, and if it turns out the motherboard really CAN support 16GB DIMMs, which will likely exist by then, I may have upgraded to 32GB too.
And of course, any time you buy something expensive, you're gonna tend to have some reservations or regrets. I still sorta wonder if I shouldn't have turned to the dark side, gotten a machine with significantly more CPU and GPU power and the ability to run them at capacity for a long time without overheating, and probably a second internal drive bay, spent $500 less on it, and done a Hackintosh. But I am not comfortable with the Hackintosh concept. As of yet, I am able to muddle through with what Apple's willing to sell, but I really do worry about their ever-increasing emphasis on thin machines that lack ports I actively use.
