I get that you can make a stir plate, but if I had to pick one over the other it would be the stir plate.I made one my self but I certainly did not have the parts lying around, just hard drive magnets. The rest I spent about40 bucks at radio shack buying.
From a stand point if making better beer I'd go stater/plate.
I save maybe fifty cents a pound on grain buying in bulk with my mill. I'll have to make nearly 20 five gallon batches just to pay it off. The mill is more of a control thing for me, if you can't get in on a group buy the cost savings are not that great.
Edit: what the prior poster is saying is with a stir plate you can make smaller starters than with agitating therefore also saving money on dme and yeast. See mrmalty.com