major difference between code in webmethod normal code

wuld like to know difference between  code in webmethod in web service( asmx), normal aspx.cs code & in webmethod in aspx.cs

in terms of

1) processing speed / time

2) resources utilized

3) page refreshing

4) anything else.....

eg if  i need to populate a dropdown from databse & use it in various  events

which wuld be best or all same