The following question will help you determine which plastic to use for any given project:
Which filament will print in your printer?
What will the print be used for?
What will the print be expected to withstand?
Will warping present a problem in the future of the print?
Is the filament safe for the end user such as children?
Is the filament easily assessable?
Is the cost of the print reasonable?
Some printers are only capable of printing with one type of filament, ABS OR PLA such as the Makerbot Replicator 2 that only uses PLA. While other 3D Printers can print with both ABS AND PLA such as the Afinia 3D Printer and the Cube by Cubify.
If your printer is capable of printing both ABS AND PLA, then it is important to choose which is best suited for the planned project by asking these questions:
Will the printed object be in the sun?
ABS is negatively affected by UV rays, whereas PLA can withstand long term exposure. (See Thing 25638 at thingiverse.com)
Will the printed object be exposed to heat?
ABS can withstand higher temperatures than PLA that will loose form when left in a heated environment like a car. Example: Hot coffee in a mug (See Thing 51835 at thingiverse.com) made from PLA would warp.
Does the printed object need to be environmentally friendly?
ABS is petroleum based and may present health issues when experiencing high temperatures. On the other hand PLA is plant based. Example: Hot coffee in a cup (See Thing 30740 at thingiverse.com) made of ABS may leech chemicals
Does the printed object need to be flexible?
Therefore, when choosing the filament to print a 3D object, remember that ABS is a stronger, more durable, less expensive filament (Octave ABS 2.2 lb filament on Amazon, $31.00). However, it is petroleum based. PLA is environmentally friendly and easier to print with, but is more expensive (SainSmart PLA 2.2 lb. filament on Amazon, $36.88) and does not hold up well in heated environments.