AJAX Calls to ASMX or ASPX web methods or ASPX classic with query string or?

What is the most efficient way of calling some business logic from javascript on the client side using AJAX?

1) call a web service?

2) call an asp.net page with web methods?

when i call a web method the entire page is instanced: page init...page load...requested web method?

3) call an asp.net page with query string? eg. page.aspx?func=funcname&params=1

then in the page load ... switch functionname

the overhead of entering in page load and make the switch ca be overlooked?

4)other solutions?