BOTH! It will increase your chances of getting a review. 76% of customers will leave a review for a business when asked.
If we had to choose the best option, text messages will more than likely get you new reviews more quickly. Customers will more than likely click on the link in an SMS text than in an email. An SMS text has an average open rate of 98% while email has an average open rate of 15-25%.
If you do not have a cell phone number and email address for customers, then you can still send a review request to whatever contact information you have. It will increase your chances of getting a review.