I am making this article in response to an overwhelming number of times I have to answer this question. I am going attempt to make this posting grow to reflect more suspension items in Tein's lineup. Right now, it should give you a little insight into which direction you should go!
 

Basics - For the customer who wants to lower the car, improve handling, and retain a relatively nice ride as compared to stock. Most times the ride is comparable to stock. Over certain roads a difference can be felt. This suspension makes the car feel more attached to the road. Springs are firmer than stock but cannot be considered a "race" setup. We often get people stating they want lowering, firmer ride, but do not want a track suspension. The Basics will generally work well for this customer!
 
SS - Same thing as Basics with the ability to adjust stiffness. The springs are the same as well, so adjustability is on the damper compression/rebound rate. There are 4 reasons to get the SS over the basic.  
1- You have extra money to spend and always get the more expensive thing.  
2- You are really concerned about ride, so you want to soften it up making it closer to stock.  
3- You are really interested in performance, you want to be able to dial in the shocks where you feel they handle the best.  
4- You want your gadgets to have more tweak-ability.  
 
CST (Comfort Sport) - This kit is similar to the SS on the surface, but a closer look reveals that there are significant differences. The springs are softer, shock are mono-tube, and the MSV piston valve technology helps make this kit ride really smooth and soft. If you want you car to feel like you are riding on a pillow, this is the one to get. Lowering is generally significant too although it is limited because there is no full-length adjustment. This kit is ideal for the VIP-inspired ride that will be daily driven. Upper mounts are included in this kit, and they have rubber bushings instead of rigid bearings for better road vibration isolation.
 
FLEX - FLEX are for the person who wants maximum handling with a ride that is still acceptable on the street. These have a higher spring rate than the Basic/SS. These shocks and springs are designed for a higher performance level. This also means they will ride firmer than the others. Upper pillow mounts are included, which increase responsiveness and linear feel of suspension, but also make the ride feel harsher (similar to effect of installing low profile tires). The Flex also have a unique lower bracket that facilitates changing of the damper length. This gives more height options and allows for spring preload. If maximum lowering (3"+) is your goal, the FLEX will work. These adjustments also allow for greater track tuning.
 
EDFC - Simply allows you to adjust the firmness of the damper from inside the car. The kit consists of an electronic controller and 4 stepping motors. The motors mount on top of the Tein damper and is wired to the controller allowing adjustment of the suspension from the driver's seat on the fly! The kit is complete and no other items need to be purchased for installation. Not always compatible with all cars and all adjustable dampers*.  
 
* On the TSX it works with the FLEX and CT, but not SS due to clearance issues on the car.
* On the 99-03 TL and 01-03 CL the EDFC will work with SS and SS-P kits.  
* On the 04-08 TL the EDFC works on SS kits but requires the removal of the oem front strut bar. Fastline Performance is currently at work prototyping a replacement brace for this application that will allow edfc installation.
 
 
 
Another comment on SS and Basics compared to stock:
The ride on either kit is very close to stock. Because of tuned rebound rates, the handling is much improved. This combined with higher compression rates and matched coil springs, more road irregularities can be felt. This means that while the softness of the ride is scarcely different than stock, the actual road bumps and cracks can be felt a little more. Over larger bumps, the suspension will not travel as much, creating a bump in the ride, but the car returns to neutral state quickly. This provides a feeling of stability over stock suspension.
 
So, will you feel a difference? Yes. Will it feel firmer than stock? Yes. Will the ride quality be the same as stock? Yes.
 
For a more stock feeling ride, the SS allows you to tune the dampers softer giving more cushiness.