Higher duro (harder): Boardside
Lower duro (softer): Roadside
The reason, as stated above and as commonly accepted, is that the BS bushing holds your weight more than the RS bushing.
It can be different if you have a softer Eliminator-style (stim) bushing BS, and a harder barrel/cone RS. The stim shape has a lot of material to compress, and can seem harder even though it's not." -Silverfish forum
If anything just go ahead and try it out and see how your board feels! Preference my friend, preference.