云共享数据库Web应用程序中的主键类型

i'm building a cloud web app with asp mvc and mssql. Clients will share the same database. I still havent decided what type my primary key will be. Should i use GUID or Bigint? with bigint, i'm worried about scalabilty. with GUID, i'm afraid of performance. What is the best practice here? What big cloud website like stackoverflow use as primary key? Please shed me some light.i'm building a cloud web app with asp mvc and m