This is an interesting question and one whose answer may be more influenced by a person’s lifestyle and food choices.
Until recently, products that contained both pre and probiotics weren’t an option… there were issues with the prebiotics and probiotics being combined in the same capsule and for that type of combination to have the ability to survive various gut conditions. Therefore, the option was left to the specific combination, number of different strains and number of CFU’s in a probiotic formula.
If you eat a good amount of fiber – both plant and grain based – then the good bacteria in your gut are most likely well fed. Also, if you feel that your foods are being digested and broken down well, that would be further evidence that your good bacteria are getting the food they need to prosper. This couples with daily ‘exercise’ or at least a non-sedentary lifestyle would suggest that mega (10 or more strain and 20 billion or more CFU) probiotic would most likely be your best choice.
On the other hand if you’re not a big whole grain, fruit and veggie eater and don’t move about much on a daily basis, then your gut flora would more likely appreciate some prebiotic support – and a good prebiotic and probiotic combination may serve your needs best.