Either will be OK for about a year or less. My Golds went from about 1" to 4" in less than 6 months, at which point they went outside to the pond. They grow REALLY fast.
Similar, but smaller fish that can stay in this tank size:
Long Finned Rosy Barbs have something of the look of the longer finned Golds.
Platies and Swords are being bred with the colors you might like in a Goldfish.
I like the shorter, wider tank for aquascaping. It is difficult to 'scape something as narrow as 12" and make it look like it has some depth.
Depending on the manufacturer:
A 40 breeder is 36 x 16 x 16 (plus or minus an inch)
A standard 55 is 48 x 12 x 21 (plus or minus and inch)
There is a tank that is 4' long, but deeper (back to front) than the 55, and not so tall. I really like this one, and the 40 breeder.
For the Comet and similar faster swimming Golds, I would go with the longer tank.
For the Golds with the deformed spines, it does not matter so much, they are slower swimming, so do not need the length of the tank.
I agree that for aquascaping the wider one is the way to go.
I am trying to make caves, pathways, valleys, mountains,etc.. and I can't really do any of that because the aquarium is too narrow(12 inches)
Also, the chair I sit down is right up close to it to see the small red cherry shrimp,etc.. and I basically can only see 50% of the aquarium because its too long.
A few years back, I tried to get a good idea on a large Acrylic Aquarium on Craigslist. I always hear stories about the guy who gets the 150 Acrylic tank for like $30.
Anyway, I found most people charge top dollar in my area for these tanks. Anyway, no one would never write down the dimensions.
You can have a 150 gallon tank that is 96 inches long or 48 inches long but wider and higher. There is a big difference. Sometimes, I did a calculation and found out that their 150 gallon tank was only like 110 gallons. Anyway, its shocking how about 95% of the people don't tell you the LxWxH of the aquarium they are selling.
I find it a lot harder to find the wider tanks than the longer tanks.
I had my Goldfish for like 5 years in a 10 gallon tank. Last year, I upgraded him to a 20 gallon tank. It still seems too small so I am thinking about upgrading him something closer to a 55 gallon tank. I think its a comet goldfish. He has grown a lot since I put him into the 20 gallon tank.