It is best to base the level of service requested upon your own ability to work in WordPress. If you know your way around in a WordPress Installation and are already comfortable using tools such as FTP clients and phpMyAdmin, then depending on your problem, the Brief Response may be all you need.
If you are familiar with FTP clients and phpMyAdmin but would be more comfortable with a step by step guide, then a Detailed Reply may be more suited to you.
However, if you are non-technical and know that you will likely need someone to do the work for you, then Brief Repairs or Extended Repairs would be good choices.
For complex issues or custom programming, the Major Repair would apply.
You can always increase your service level. If you start lower cost services and decide that you need more help, you can always upgrade to more advanced services without spending more than if you had chosen the advanced services in the beginning.