Your solution loops {number of collections} * {number of required items} * {number of items per collection} times.

jhourcle's solution loops {number of collections} * ( {number of items per collection} + {number of required items} ) times.

We weren't given any numbers, but I bet your code will get execute many more times than jhourcle's, making it much slower.

Update: Here are some very conservative sample numbers:

Number of collections:          5
Number of items per collection: 8
Number of items required:       10

artist:   5*8*10   = 400 iterations
jhourcle: 5*(8+10) =  90 iterations
[download]

Update: The OP gave some numbers.

Number of collections:          5  (Conservative guess.)
Number of items per collection: 12 (Middle of provided "5 to 30".)
Number of items required:       6  (Provided.)

artist:   5*12*6   = 360 iterations
jhourcle: 5*(12+6) =  90 iterations
[download]