all comes down to how low you want to go, springs will give you a fixed drop, where as coilovers give you an adjustable option, this means you can then have the rear lower than the front etc.
the think to keep in mind with just changing the springs is that, these springs aren't going to be designed to work with the dampeners already on the car (eibach pro sport are meant work well with VW Sport OEM dampeners though) as a result changing the springs can result in extra wear on the dampeners and ultimately in them failing prematurely, on the other hand coilovers (genuine coilovers (which technically you can't get for a MK4 given that the rear suspension is trailing beam)) are matched as in the spring is matched to work with the dampening that the spring "coils over" this results in a more predictable spring rate /dampening etc, and for the same travel should be smoother than shocks and springs (more progressive). As previously said coilovers also let you adjust the suspension as you wish (which in motorsport would be tweaking for different tracks) but in the modding scene it often means just getting the ride height and walking away (a waste of coil overs IMO) but often the only way to achieve 45mm + drops
also keep in mind like everything in life, the amount you spend with affect the quality of what you get, so £150 coilovers aren't going to be as good as £1000 one's however if you're looking to purely change the ride height they will both do the job.
I'm personally running eibach pro sport springs and dampeners (which are "matched") as i only wanted a 20-30mm drop to produce a similar ride height to the anniversary, he eibach setup also offers probably the best compromise between ride comfort and handling (for springs)
if you're thinking of going lower, then look at steve_pd's pics as he's on about 45-50mm drop (coilovers)
more info on lowering can be found
http://forums.vwvortex.com/zerothread?id=1835863&postid=18980504#18980504
