Autocomplete and suggestion features serve different, though similar, purposes.
Autocomplete is there to make it faster for you to complete your search. If you for example want to search for "wikipedia puppies", and start typing, you will get an intermediate stage of:
You can see that the autocomplete has provided me the option of completing "wiki" with "wikipedia". If I were to then press
tab I would get the following:
So in this case I've used autocomplete to complete a word. It could also have been a phrase, as is the case below:
suggestion on the other hand simply shows probabilistic searches matching against searches that other people have made. This is in a way an autocomplete function, but it also provides options and more of a complete search than partially finishing your sentence. It also provides more alternatives, that a simple
tab to autocomplete. You can also think of the autocomplete as a fast selection of the most probable suggestion.