Having considered just about every configuration except the base 21" and the BTO GPU (not a gamer), I think I've finally settled on the base 27" with Fusion Drive.
For what it matters, here's my thinking:
1) I'm replacing a 2009 i7 iMac which actually performs fine for me from a CPU standpoint, and the 2.9GHz i5 in the new machine should be in the same ballpark. However, I've been booting from a variety of FW800 SSDs since I got the machine, and while this is faster and more responsive than the internal HDD, it will still be nice to eliminate the awkward setup and management hassles with the Fusion Drive.
2) The biggest single reason for the upgrade is Thunderbolt - I'm a professional photographer and my photo library lives on a 4TB RAID that can only connect via FW800 now. My working versions of my current year photos are on the internal drive for speed, but I'll be getting the Lacie eSATA Hub to hook up the RAID so that I can work directly from there going forward.
3) USB3 - I have one USB3 enclosure already; it currently houses my iTunes media, but I'll probably move that data to the internal drive and use the USB3 drive as my nightly clone of the internal instead.
4) Less reflective screen - while my screen doesn't face a window, sunlight coming into the room still causes a fair number of reflection issues, so this will be a welcome change.
Why this specific model? I had been leaning toward a maxed-out 21.5" i7, but came to the conclusion that the i7 probably isn't necessary (I have iStat Menus running, and just about the only thing that maxes out the virtual cores is Handbrake). Once I came to that conclusion, it was really down to two virtually identical machines - 2.9GHz i5 21.5 vs 27, with the latter having a slightly better GPU. There is a $300 price difference, but since I'll be able to buy 3rd party RAM and install it myself ($50) rather than pay Apple for 16GB in the 21.5" ($200), it's really only $150 for the bigger screen and better GPU. That seems like a good trade to me.
Of course, that doesn't mean that I don't still ponder getting the 21.5" instead just so I can get it sooner