Personally, I believe more in the prebiotic approach. The reason is that probiotics are live microorganisms that are taken orally as supplements and therefore may be considered foreign to the host. Knowing how the vertebrate immune system works and that the interaction between any microorganism and its host is dynamic and dependent on the particular organism and the particular host, under a particular set of circumstances, it becomes quite clear that the outcome of the use of any probiotic cannot really be predicted. Prebiotics on the other hand, work by stimulating indegenous microflora which then exert their beneficial effectson the host. Thus all the disadvantages or side effects that may be associated with the foreigness of the probiotic to the host may be avoided. In many cases also, prebiotics may be cheaper.

Dear Mohammad

I think when you want to evaluate pre or pobiotic efficiency in a new species, the first task is selection of the dietary inclusion level based on the results reported for close species; specially in case of prebiotic that the dosages should be selected carefully, as higher doses results in negative effects on host.

when you determined the levels of inclusion then you can measure the efficiency of selected pro or prebiotic based on the changes of intestinal microbiota . Indeed an effective pro or prebitic should change the balance of microbial communities of Intestine toward potentially beneficial species; This  is the most important factor that per se determines the efficiency of pro or prebiotic on host growth performance, immune response and resistance

I think is better to evaluate symbiotics, because this approach generates a complete technology, depending less on interactions with normal flora.  Obiously, like Mr Hossein says, you need to carefully evaluate inclusion levels.