Should registration be optional on an e-commerce site?

I am developing an e-commerce web site which will sell tourism services (airport transfer, accommodation, excursions, etc.). I am on a path here that I need to decide if I will provide a membership system or not.

From the user's perspective, the following two approach is possible:

• Registration: A user should register for an account and then she can purchase online. Then, for the next time she can come to the web site, log in and purchase w/o entering the personal details again. Also, the user will be provided with a portal in order for her to keep her personal information up to date. Also, this approach makes it easy to provide a management portal for making changes on current reservations. One downside of this approach is registration is annoying and can scare the user away. If I choose this road, I will provide a way to use OAuth but it still can be overwhelming.

• W/O Registration: A user can come to the web site and purchase a service or product without needing to register or log in. If the user needs to purchase another time, she needs to enter her details from scratch. These two purchases are completely separate from each other. They won't have a relationship and one cannot affect another.

I've considered mixing the two options and provide a way to choose but I do not have time for this. I need to choose one and go from there.

Do you support either of the above approaches or have a different idea?