Sounds as though you have settled on the rMBP, but perhaps another approach you could think about is a 2009/10 Mac Pro. I have two Mac Pro models at work, both bought as refurbs from Apple and they are workhorses. I have 5 HHDs in each with 32GB RAM, "FAST". I have no interest in the "Garbage Can" 2013 Mac Pro model but wanted another MP for home use, so I'd been following a couple of machines for a while on Kijiji. One in particular in Toronto, and after tracking it for about three weeks I got in touch with the owner and made a deal. That's my first experience with buying any sort of computer equipment on Kijiji, but it worked out very well. For $700 I have exactly what I wanted(2009) and will now add more RAM and a couple of HDDs as well. Now I can take work home and have exactly the same setup there by cloning the HDD on my work MP. I don't know if you are at all interested in a tower, but you would definitely get more bang for your buck in my opinion, as well as greater expandability. Hope I haven't muddied the waters further Michael, just trying to add to the list of possibilities.
You know everything can look like something else. So itís round and has a recessed top, it is also TINY and wouldnít hold more than a few pieces of wadded up paper. To me it looks like a little jet engine, or a mini cray. Bottom line it is basically silent, generates very little warm air, and it is crazy fast (launches Photoshop in under 2 seconds, internal SSD write speed is 975 MP/s, getting 700 MB/s from a LaCie Big5TB set up as a raid 0 (TB 1, a TB 2 cabinet will probably be faster), and over 500 MB/s with two 4 drive raid 5's set up using LaCie Thunderbolt->eSata connection. I tried 4 different eSata cards in my 2010 Mac Pro and all were completely flakey, ended up stuck with FW800. In 2 weeks not a hiccup with eSata this way. The great part is they are 10 feet away from my computer and I can't even hear them spinning. The new Mac Pro is incredibly sweet and blows my decked out 2010 12 core mac Pro away ( 64gigs, PCI based SSD start up drive and 4 7200 rpm raid 0) . saving my very large image files used to take 10 - 20 seconds, now takes less than half that, and reading them in when launching is more like 1/4th the old time ... (sry, getting off topic and I stay away from the computer section on websites to avoid getting into conversations like this. I apologize ... back to the topic.)
And you are right, there is plenty of mileage to be had from older Mac Pro's especially if you can invest a little into them. The OP is interested in a second machine, and while a second tower may work well for you at home, I don't think the speed benefits you may be able to eak out of it out way the other benefits. A brand new Mac mini is probably cheaper than a used 2009 Mac Pro, base specs are only slightly slower. A MacBook Pro specs out faster, and both of those accept thunderbolt storage with is faster than the internal based sata of the old tower. The only reason to put hard drives inside the box are for access speed, now that is no longer necessary keeping all main storage on secondary TB or eSata cabinets is a much better way to go. Your storage is now no longer computer specific.
The 2009 (or better yet 2010) Mac Pro option is certainly worth considering if having the portability or needing the small size at home isn't critical. The main advantage would be you can put in up to 64gb of RAM(although not cheap), you can even run a PCI based SSD drive in there and boot from that. While this isnít as fast as the speed of the MacBook Pro or Mac Pro built in SSD, itís faster the a 4 drive raid 0 internal to the Mac Pro. (As far as overall storage, internal drive storage isnít a plus for the tower because the external TB raid is faster than you can get inside the tower). By the time you buy a used Mac Pro, add 32 or more gigs of RAM, add 4 hard drives and setup a high speed raid ... hmm, maybe the Mac Mini even though theoretically a little bit slower is suddenly much more economical.
In my case I know "bench marks" show the 2010 Mac Pro to be faster than my "early" 2013 Macbook Pro, but when I'm running LR and PS using external TB storage my MacBook Pro is more responsive than my decked out 2010 Mac Pro. I assume it's mainly about the amazingly fast integrated SSD storage. I would have ditched the Mac Pro last year except I have 2 30" displays ...