First, let me comment on the various data access options:
- PI Web API: Easy-to-use cross-platform/multi-user programmatic access to the PI System. It uses a RESTful interaction model. Technology is under active development.
- PI AF SDK: Since it is a .NET library, easy integration with the .NET framework. Technology is also under active development.
- PI SDK, PI Web Service, and PI OLEDB: I would not encourage using these technologies because there are newer, more robust alternatives (above options). Also see: Deprecating the PI SDK
While PI AF SDK offers the most functionality, PI Web API is undergoing a lot of development and has recently added Ad-Hoc read/write bulk calls to speed up data retrieval (see change log for the 2015 R2 version). Both products (with the latest releases) allow you to send bulk summary calls to the server where the calculation is performed server-side (since you are interested in average data).
Another point to note: PI Web API is built on top of the AF SDK, which is designed to be run within the context of a single user. To support multiple simultaneous users, PI Web API hosts multiple instances of the AF SDK. These instances of the AF SDK are provisioned automatically by PI Web API.
I also encourage you to check out Lonnie's comment in a previous discussion about other functional differences between PI AF SDK and PI Web API: What is the best way to connect PI AF with .net web portal and why ?