careful with buying them on ebay, they have a specification for surface heat evenness, at least the good ones, used units that are cleaned up and look good might not have this specified uniformity. They will tell you something along the lines of "the specified surface heat differential minimum and maximums are at most some value of C when a thermal camera is placed x-inches from the surface"
So if you really want good SMD joints, I think its a big reason why people use IR heating, because you can tell the source is uniform.
The way to check it is using a thermal camera, so if you do not have one, using it for things other then heating beakers is not a good idea for what you are interested in. I got one once, thought it was nice, it cleaned up good, looked great, but the surface was un-even so it I don't think it was suitable for doing something like a thin heat sensitive PCB. The replacement top for it cost like 5x the price of the hot plate.
This is one of the parameters you are paying for, how long it stays uniform on the surface to specification.