howard, here's an excerpt from the book "starting strength" by mark rippetoe & lon kilgore, page 61, somebody posted a link to the pdf version a while ago
"squatting" in a smith machine is an oxymoron. a squat cannot be performed on a smith machine.... sorry. and a leg press machine - the "hip sled" - may be even less useful. both of these devices restrict movement in body segments that normally adjust position during a squat, thus restricting the expression of normal biomechanics. the leg press is particularly heinous in that it allows the use of huge weights.... a 1000 lb leg press is as irrelevant as a 500 lb quartersquat.
because of your back you may stick to front squats though