[kekie]

What's the difference? Which one should I use?

[mjc123]

Try this reference:

http://pubs.acs.org/doi/abs/10.1021/ed077p1366

Abstract (I couldn't access the full text):
The term "periplanar" was coined by Klyne and Prelog in 1960 to define a flexible torsional angle of 0 ± 30° (syn) or 180 ± 30° (anti). The prefix peri, derived from the Greek for "near", was chosen to make the meaning "approximately planar". However, the current common usage of syn and antiperiplanar is planar, which is incorrect. In the interests of proper language, we suggest that future authors instead use "syn-coplanar" and "anti-coplanar".