I think if I were in this situation I would first ask "do I really need two computers?"
For me, the answer is yes because I record some Sirius program content at home when I am not there, and I have an old Mini doing that job.
But for most people, one computer is just fine. For the sake of this particular argument, I will assume this would work for you as well.
Starting with your needs and budget, I would look very strongly at the 15.4" RMBP as the base computer. I would then add to that a good 32" HDTV, not that you need the connectivity and HD tuner (although that never hurts), but because a 1080p (1920x1080) TV makes a really good (and big) secondary (or primary) monitor, and it is less expensive. The HDTV and computer monitor have all but finally converged, and the best choice these days is a HDTV which can do pretty much anything a computer monitor can do, and more, for less. It won't have all of the conventional computer screen resolutions, but the ones it offers work really well with all the available Mac resolutions.
For instance, I have a 2011 MBA that I use with Aperture, and I got a $5 Thunderbolt to HDMI adapter from monoprice.com, that lets me use a 1080p Vizio 32" HDTV as a second 1920x1080 screen. I could not want anything more than that.
I did an extensive field test of which HDTV/monitors look good close up, and the Vizio seems to have the best "pixel fill factor", which is the ability to use the highest percentage of the area designated for a particular pixel dedicated to actual pixel rather than visible surrounding support infrastructure. Put another way, on 32" monitors by other manufacturers you can see the pixels from an arm's length away, while you can't on Vizio or Sony (but Vizio is best in this regard, and high value at a great price).
The RMBP is smart enough to know how to format the resolution for the second screen automatically, once connected.
I consider this the best setup. You don't have to worry about file transfers between two computers, you have a second hi-rez monitor that completely fills your view from a normal "computer monitor" distance, you have a second screen (on the RMBP) for palettes, and you can push the budget towards more RAM and a bigger SSD for the RMBP (the 1080p monitor will be under $400). This gives you the best "portable" mode (the RMBP) as well as the best "desktop" mode. This will cost you less than $3K.
If you get a laptop and iMac, $3500 will be enough for starters; $3800 will not be enough to fully trick out both; you will have a single smaller monitor in "desktop" mode, plus you have to manage files and apps between the two computers. And if you really want an iMac, it makes sense to wait until Sept-Oct when the refresh occurs.
