You could implement both. I think the main thing to remember is not to force it unto the user, that will only create a bad experience.
You could make a "Like us? Review us!" section on your page, but make sure your call2action draws more attention. You can also make it on your confirmation page with the same train of thought.
You can send a request for a review about 3 days after (is to my findings a good amount of time). Make it a pretty mail, don't beg.
If it's possible, you could grab the emailadresses from those who've reviewed so you don't mail them aft er those few days, that would be considered enoying.
An alternative to that might be added to that mail something along the lines of "Allready reviewed? Great!"
Another moment you could ask, is after you helped someone (via phone, via email) and that person says "your service is great", you can ask "Great to hear, may we ask you write a review with that enthousiasm so we can show that we're good".
Don't expect 100% of the users to write a review, then you will only get disapointed