As advised above, stick your camera on a tripod and run some cheap film through it, taking pics of something consistent (brick wall is a usual subject for this kind of thing, perhaps one set of images close up and one set further away), with shots at each aperture on each of the two lenses (and make sure you keep a note of what you're doing!). You should get a pretty good idea of sharpness and contrast in centre/corners at different apertures and distances. It should also highlight any problems with the diaphragms etc. A couple of rolls of Poundland Agfa and Asda development will cost you £5 max, and it's a good way of getting a feel for how your lenses perform, both for comparison purposes, but after for when you decide which one to keep.
If all is pretty much equal, I would keep the faster f2.8 one and sell the other, which given the above comments is probably also newer, and therefore likely to have better coatings too. There are of course other measures of lens quality which might not be apparent from that test (eg flare characteristics), but they would be harder to compare without more extensive testing.
Minolta lens by john_shears, on Flickr
