I'd say it ultimately comes down to what you are trying to do, for trading or otherwise.
I would start by visualizing or even sketching out on a piece of paper just how you're going to arrange all your charts and other apps among the displays. Ergonomics and usability ought to be some of the key factors for you to consider.
In terms of total screen real estate, 6x20"s beat 1x30" + 2x24" hands down, by 32%:
6x1600x1200 = 11.520 Megapixels
2560x1600 + 2x1920x1200 = 8.704 MP
But if you want the wow / sexy factor or plan to watch DVDs on the 30" after hours or maybe just like to be an early adopter, then go wide-screen, with the big ones.
Two quick comments on your price comparison. First, it's incomplete without also taking into account the cost of all the video cards required for each setup. In part, for the 30", you'd need a more expensive dual-link DVI card (not to be confused with a merely dual-DVI card).
Second, wouldn't you need a suitable stand for the 2nd setup as well? Unless you're thinking of just putting all 3 LCDs on top of your desk... however, that might be way more horizontal span than optimal, in terms of your field of vision.