I think you are fine either way you go. The only reason I ask about the plant number is I see a lot of guys put 3 plants in 25 gal container and then use a large tomato cage to stake them up.
15 gal pots are roughly 17 1/2 x 15 inchs tall - a 55 gal drum is 34.5 inches tall x 24 inches wide. When you half the drum it is the same height as the 15 gal pot but a little wider. Now the question is how many plants are you going to grow in each container. Most likely 1 in the 15 gallon...
I think your issue is not going to be size of the pots but the weight of them after you have soil and plants in them, you are not going to be able to move 1/2 55 gallon barrels with ease if you had too. I would go with the 15 gal.