Personally, I prefer a progressive spring. Most sleeve type coil-overs are non-progressive, to illustrate, I do this in photoshop:
all springs have a "k" constant that represents, essentially, the "flexibility" of the spring. A progressive spring (left) has different levels of resistance throughout the coil, this allows for more or less play - it's these springs that give you a comfortable ride... and put less strain on your shocks and other parts.
For the sake of coil-overs, the springs are mostly uniform like the spring on the right, but the higher end coil-overs have things such as helper springs or even have a progressive spring already installed and fitted. e-bay sleeves do no have progressive springs or helper springs as it would be difficult to make a universal set specific to your car. (I don't care what you say, those e-bay sleeves are just about universal and not car specific... the difference between the v6 and 4cy springs SHOULD be different in spring flexibility (k) but is not when it comes to these e-bay kits... they just don't put enough research in their products, thats why they're cheap)
Other coil-overs fix the "bounce" problem you see in most cars equipped with e-bay style short sleeves by using the dampening force in the shock it's paired with or by using a longer spring and shock assembly (or push the threads on the sleeve all the way to the BOTTOM of the shock). e-bay sleeves do not come with a shock/strut assembly so they obviously cannot fix the bounce this way. They also sit where the stock spring is so the threads are higher up than on real coil-overs giving you a smaller space for the spring... thats why they're so short.
I mean, it's your money you can do what you want with it. I would rather prefer something that feels smoother and puts less strain on the rest of your parts (esp shocks) and can adapt to different situations... and not only that, but something that has been tested over-and over for safety.
Notes:
H&R adjustables use a progressive spring:
These Blisten adjustables use a longer coil:
These Tiens use both a longer spring with lower threads and a helper spring up top:
These Eibach adjustables use a helper spring:
If you do end up using sleeve-style coil-overs, I'd only go with Ground Control because they have Eibach technology in their products.