General RAM query

A general enquiry rather than specific to Inspiron laptops:

I am looking to replace my 17 R SE Inspiron which no longer charges and for which I have been unable to source a (working) new battery.

When specifying RAM, is there any benefit in using both rather than one slot - i.e. are 2 x 4Gb modules better in any way than 1 x 8Gb? 

Initially, I am thinking of an 8Gb RAM model, but having the option of adding a further 8 (without having to use 2 new RAM modules) - which would make 1 x 8Gb the obvious choice, but unless this is in some way inferior to having 2 x 4Gb, I cannot see why the latter would be offered as a configuration option.