The difference between PVC and ABS pipe
The two most common types of pipe for process pipework are ABS and PVC (also more accurately referred to as PVC-U where the "U" stands for "unplasticized" - PVC which is "plasticized" is made into tarpaulins, flexible hose and other non-rigid items). The ease of cutting and assembling these systems, in addition to their great resistance to abrasion, chemicals and the wide range of fittings available make them the systems of choice.
PVC pipe systems are the cheaper of the two, and by far the most widely used (probably by a factor of about 10 :1). The main areas where ABS is preferred over PVC are those which involve cold liquids and slurries or where a degree of impact resistance is required. ABS is operational down to -40 degrees centigrade, whereas PVC can start to become prone to shattering on impact as temperatures approach freezing. That having been said, PVC pipe is widely used outside, with fluids of near 0 degrees C flowing through it, and is fine, as long as there is no significant vibration or risk of impact.
Both ABS and PVC Pipe have much the same upper operating temperature of 70 and 60 degrees C respectively, with a similar curve in terms of pressure rating drop off as the temperature gets higher.
Both ABS and PVC systems are available as WRAS approved for potable water, however with PVC systems, there is sometimes another option for pipe and fittings that have not had WRAS approval. This doesn't mean that they have failed an approval, just that they have not been presented for it. WRAS approval (which is similar to KIWA approval) is a VERY expensive process and some manufacturers do not regard it as necessary for their products. As a result of the cost of WRAS approval, not WRAS marked products are usually cheaper. Non WRAS pipe is very widely used in water treatment, aquaculture and agriculture, factory processes etc. ...in fact just about everywhere that doesn't involve connection to a tap that will be used for drinking water.
In summary, if in doubt, and the application does not involve freezing temperatures, you are probably best building your plastic pipe system with PVC.