Mostly, wheel spacers would only create the necessary distance on the inner portion of the wheel for clearance to solve the clearance problem of brakes, tie rods and other suspension components. If you plan to keep your current wheels and increase the offset of your car for enhancing the handling performance or exterior looks, then wheel spacers will be your ideal choice. On the other hand, wheel adapters can not only do what spacers do but also can change the bolt pattern (and possibly center bore) to allow you to mount wheels that are not made for your car. If you intend to convert PCD/CB for fitting the wheels, then you should make a selection of wheel adapters.
In addition, wheel spacers are used for a couple of reasons. When wheels do not have enough offset. The wheel spacers would compensate for proper offset by pushing the wheels away from the vehicle hub. Having your wheel brought out the desired distance allows for mounting larger brake calipers without wheel scratching. Moreover, a hub-centric wheel spacer can transform a lug-centric wheel (one that centers by fastening the wheel lugs rather than a hub-specific mounting surface) into a hub-centric wheel. A wheel spacer is usually a single piece, thickness less than 20mm (0.8 inches). They are sandwiched by the wheel and vehicle hub. To be considered safe, they will need extended wheel bolts to achieve proper thread engagement.
Wheel adapters have all the functions of spacers. Apart from adding the clearance between wheels and vehicle hub, they can change the bolt pattern (and possibly center bore) to allow you to mount wheels that are not designed for your car. One of the common uses for wheel adapters is PCD conversion such as 5×130 to 5×112, 6×139.7 to 5×127, 5×120 to 5×114.3.
Another application is CB conversion. For example, BMW G20 uses F30 wheels. The center diameter of the new BMW G20 is 66.6mm and the PCD is 5×112. By installing a wheel adapter on the G20 which has a 66.6mm hub-centric ring and 5×112 bolt pattern on the hub side and the wheel mounting side of the adapter will have a 72.5mm hub-centric ring and matching studs in a 5×120 bolt pattern on it. This allows F30 wheels to be mounted on the G20. Wheel adapters are usually thicker that are built with their own lug studs/nuts. The adapters lug nuts to the factory studs, then the wheel is secured to the studs on the adapter with the stock lug nuts. Two sets of bolts pattern hold the adapter in place. The adapters are usually made to offer space greater than 20mm (0.8 inches).
Here is a brief explanation of the wheel spacers and wheel adapters. Hope this article can help you know which parts suit your car. Every vehicle is different, so please make sure which is suitable for your modifications. And, if you meet any problem with the selection, we have a support team that consists of professionals ready to help.