heh, man, i wish i had your problems. here i am with a single 20" dell monitor (cause its cheaper) thinking im in hog heaven! but i always think how nice/cool a 23" would be b/c it is HD. but anyway. . .i would consider the size of 2 monitors, and turning your head all of the time. its one thing to run dual 17" monitors when you just need to run a lot of things at once and want to have quick access to each of them. but with a single 30" monitor you can do A LOT (thats just a guess, ive never used one outside of the apple store) but b/w gobs of screen real estate and expose i think a 30incher would be better because everything would be in front of you, instead of infront and to the side. but in the end i think its personal preferance . . . how much experience do you have using 2 monitors vs one? i used to run my 12" pb next to my 19" CRT (before the 20" dell) and i got tired of that, especially once expose came about.
. . . now, if apple would only support mulitple desktops (like a linux system with 4 desktops. . . you either know what im talking about or you dont) THAT would be cool, and i would totally go for JUST the 30"!!!!