Each type of exercise has its advantages and limitations which is why specificity of exercise is a good guide. Given specificity of exercise, if your goal is to:
increase my ability to hold heavy bags of groceries for a long time,
then holding heavy bags of groceries for progressively long periods of time would be your most direct path. This not only trains the specific muscles for the task, but it trains them in the same position where the strength is needed, and with the type of contraction needed for the the task.
Specific Muscle for the task: According to exrx:
The brachialis becomes more readily activated during isometric elbow flexion. During a dynamic elbow flexion, the biceps is more readily activated than the brachialis. (Tax et al., 1989)
So, bicep curls would not be the most efficient exercise to increase your ability to hold heavy grocery bags because not only are they targeting a different type of contraction (isotonic vs isometric), but they also do not emphasize the primary muscle needed to hold groceries isometrically, and they do not address the shoulder/scapular muscles.
Specific Position Needed: - Holding bags of groceries requires that the arms are abducted (lifted out away from the body). Biceps curls, shoulder shruggs, farmer's walks etc., are performed with the arm/shoulder/scapula positioned with the arms in neutral.
Specific Type of Muscle Contraction Needed: When you set up an exercise program with a specific goal in mind, you are right to consider the type of muscle contraction. To add to @Grohlier's answer, using the example of a bicep curl from full extension to 90 degrees, here are some of the limitations of each type of exercise:
Isontonic - The limitation with an isotonic exercise is that you are limited by the weakest point of your range.
Isometric - The limitation with isometric exercise is that strengthening only occurs at the range being worked (and about a 15 degree carry over). To strengthen through 0 to 90 degrees you would need to hold the contraction several points through the range.
Isokinetic - A third type of exercise, isokinetic, matches your maximal resistance at each point in the range with special equipment. This allows you to work each point of the range with the maximal amount of resistance. The limitation with isokinetic exercise is that the machines do not allow you to work eccentrically.
Different types of resistance exercises will strengthen targeted muscles, but matching the specificity of the exercise with a specific goal gives you the greatest success.