If you're looking to buy your first one, go with the Pro Auto Disc right off the bat. You can get longer studs and use double discs to get enough powder for rifle charge weights. I have not upgraded to this yet but most likely will at some point.
When I make mild .38 Special target loads for my daughter to shoot I use the AD on the LCT press and pick the disc that gives me the closest charge weight to what I'm after. I can live with a charge weight within a tenth or two of desired charge as long as it's not over max. The discs have fixed orifices or if you have a denser powder you may be able to use the adjustable charge bar instead of a disc. Trail Boss is low density so it takes the largest disc orifices to give me the charge weights I'm after for the .44 mag.
I got the Auto Disc when I ordered my LCT press and a friend gave me the Pro AD and Pro 1000 press. Now I need to get shell plates and more die sets, he has some spare turrets for the Pro 1000 when I do get them.
Right now I'm using the LCT for load development of 200 and 240 grain cast bullets in the .44 mag. For this process I hand weigh the charges on a RCBS 505 scale using a Lee Perfect Powder Measure to throw the charge. Once I settle on a load I will set the Pro 1000 press up for it and just run them by the hundreds. Right now the 240 SWC and 6.7 grains of Trail Boss looks promising, see this post:
http://thefiringline.com/forums/showthread.php?t=485019
I have not yet tested the loads made with the 200 grain bullets. When I do it will be the same deal, settle on a good load and set the Pro 1000 press up with the Pro AD powder measure and crank them out.
As you can see, each unit has its place and use depending on what you're looking to do with it. Were I to start afresh I would just get the Pro AD with the double disc kit right from the get-go.