Hmm, tough question. Since I have to fricking idea what your asking I'm liking the sphere. So yeah, my vote is for the sphere.
Area of effect of a Fireball in D&D?Always go with the 33,000 cubic feet volume; teach 'em to be cautious when using it in narrow dungeon passages! If targeted properly can also clear out quite a section of a dungeon as the fire flows into different rooms and corridors.(The other option is one of the many nerfs I dislike about WotC editions of D&D.)
Option 3: 33 squares on my map, "walking" drunkenly away from square in which it detonates. Unless you happen to be using it during aerial combat (lucky!).
I'm with Almostoldschool, 33,000 cubic feet but it always turns into group table math and graph paper square counting.
Unless the targets are in the air, the sphere leaves too much wastes volume.
Well, since I can't be bothered to ass about with that much math, I go with the sphere, but will use some common sense to punish the MU with backlash if he tries it in a 10" corridor.
If outdoors, even if it's next to a wall or something, I do 2" radius. I figure it's close enough and much of the extra radius from obstructions would explode upward too. Indoors I do 33 10' cubes, counting outward pretty much evenly. If it hits a door, I count around the other ways and then roll for door busting, to reflect that the door helps block the fireball. Small openings like windows get jets of flame and skip a count. So if there's two hallways, a door, and two windows I count like this: hall, hall (end 1 circuit), hall, hall, check door (let's say it fails) so count 1 past door, windcow, window (end 2 circuit). At this point we have 3 halls and 2 windows, so hall, hall, hall (end 3rd). Hall, hall, hall, window, window (end 4th). Etc. I try to be neutral with it but it is unpredictable and we have had some party-frying go down in the past. When I finish drawing the red lines it usually looks like it makes a lot of sense.