the only way to take the speeder down effectively is with the Rocket Launcher.
the empire tactic should basically be.. 2 At At s and at least 2 Rocket guys with each.. without rocket support the At-Ats wont' make it very far..
Although the counter to the rocket is speed.. the speeder can outrun the rockets at full speed.. So the speeder needs to get in and get out quickly...
it will probably come down to a game of cat and mouse.. can teh speeder get in and get the tow rope around fast enough before the rocket support can take him down..
probably the best tactic for the rebels is to send 2 ships after 1 At-At.. if 1 gets shot down there is still a very good chance the 2nd one will finish.. and considering how fast the Speeders spawn they should always be able to keep pressure on them.