the matched set is matched
you dont have to worry about them being the correct spring/dampening rate for each other... they are matched already
putting together your own set up means you need to do the legwork on your own and make sure that everything you get is properly matched up for ride height range and spring/dampening rates...
not matching them will, at best, cause the shocks to wear out faster, along with shitty handling and ride quality... at worst, you may end up breaking something, veering off the road, or into oncoming traffic and crashing killing yourself and your car load of friends along with a family of 5 in the mini van you hit, and a baby duck who was just trying to follow the mama duck across the street
another thing ive heard about a few times regarding the slip on ones... they sometimes end up rattling like crazy because the rubber ring that they give you for a bushing either doesnt fit quite right, or ends up wearing out or breaking
either setup can be good or crap... cant really put a blanket statement out there that one is better than the other...
plenty of people are using each style very successfully... more people are using each style incorrectly
i opted for a complete setup...
but i also put the time and work in to make sure that my ride height was properly set up with suspension geometry in mind and then corner balanced the car for neutral handling... i didnt get adjustable coilovers so that i could crank my ride height up and down as i saw fit... or so i could drop the hell out of my car and tuck rim...
each time that you adjust height, you should at very least get an alignment done, as the settings will change with the ride height... beyond that it also should be rebalanced
if you are not looking for adjustable coilovers so that you can tune your suspension, then just get whatever you like more or can afford (without buying garbage) because it wont make much difference... running the best / most expensive suspension components out there, wont matter if they are not set up and tuned properly...
another option if you do not plan to properly set up and tune your suspension, would be to get a set of regular lowering springs and a shock that matches the spring rate and ride height range... this generally costs even less than shocks with slip on adjustables
the real question here is what are you looking for out of your car??
do you just want it a little lower so it looks better and handles better for the street??
do you want it slammed out on the ground tucking tire/rim??
do you want it tunable for maximum handling for autocross or HPDE track days??
do you want a stiff ride so the titties on the girl in the passenger seat jiggle around like jello during an earthquake??
also please throw up some links of the equipment you are looking at... there might be a very good reason why its so much cheaper