I'm pretty sure the one that comes with the agitator assembly is so you can replace the older style transmission with the new orbital transmission, (they use a different agitator retention system, rubber o-ring on old helix transmission, 1/4" screw on orbital transmission).
If yours already has the orbital transmission then the cheaper one should be all you need - I've never seen a Maytag strip out the agitator spline!
Since you said you have the top half of the transmission off - then you must not have the new orbital transmission in your washer, (there is no top half of the orbital transmission to take off. Transmission case/body-top/bottom all one piece).
I've never tried to put an agitator from an old helix transmission on one of the newer orbital styles so I don't know if the spine is different or not. You may be able to put the agitator retention o-ring from yours onto the new orbital transmission and use your agitator but again I don't know if this would work or not.
Edited by Budget Appliance Repair, 30 April 2014 - 07:16 AM.