Both of them are very different and there are many factors that will determine what is best for you. Some considerations will be comfort versus performance, durability, adjustability and of course, price. 

Lowering springs

There are car owners who are more concerned about the aesthetics of their ride. Hence, they will opt for lower springs and worry less about adjustability or performance.

Lowering springs work just like factory springs except that they are anywhere from half an inch to 2.5-inch or more shorter than stock springs. They have a higher spring rate, and are usually designed to be used with stock shocks, so the spring rates are usually not as aggressive as performance shocks and coilovers.

Coilovers

Coilover suspension systems have become a popular staple in the automotive aftermarket. Once limited to racing teams with the research and development budget to create performance parts, these suspension systems are now widely available from most online and retail aftermarket auto part merchants.

When springs are made, they are made into coils that allow them to compress and retract. 'Coilover' is short for 'coil spring over strut' and they should not be confused with struts or independently mounted shock absorbers as they are independent and do not require extra parts

In coilovers, shock absorbers and springs are assembled as a unit prior to installation, and are replaced as a unit when the shock absorber has leaked. This provides for optimal damping - commonly referred to as 'stiffness' - without torsion loads. Some coilovers allow adjustment of ride height and 'stiffness', using a simple threaded spring perch similar to a nut.

D1 Spec coilovers feature a monotube absorber/damper - an active pressure valve - and they come with variable spring rates, customisable from 6kg to 16kg at the front and 4kg to 16kg at the rear. In addition, it features a 30-way adjustable shocks as well as urethane upper mounts. 

Another feature of D1 Spec coilover systems is that they allow for ride height to be adjusted (from 30mm to 90mm) independently of spring pre-load. With other alternatives available on the market today, vehicle ride height is adjusted by reducing or increasing the pre-load on the springs.

This primitive method leads to inconsistent spring rates that are dependent upon the vehicle ride height. Having the ride height independently adjustable from the pre-load ensures that the spring rate is always consistent, no matter how high or low the car is.

Aluminium alloys are used in producing the spring seat and locks which reduces weight yet provides a strong and secure construction. A cold-bent spring design further optimises the balance between performance and comfort.


Benefits of Monotube absorber/damper

One of the main features of D1 Spec coilovers is that they contain a monotube absorber/damper. Although monotube absorbers/dampers cost more to manufacture compared to twin-tube designs, they have several advantages over twin-tubes.Heat is one of the detrimental factors to performance and longevity of any shock absorber. Conventional twin-tubes trap the heat within the shock body, which in turn, prevent adequate heat dissipation, making them more prone to heat buildup, fade and eventual failure. On this note, monotube absorbers are air cooled which allow for better heat dissipation.

The bigger piston design in monotube absorbers allows them to withstand higher pressure and more punishment from the road, thus they are able to support vehicle weight better and offer higher dampening abilities.