In general, if the motherboard works with both types then either one will do. Keep in mind however that sometimes mixing single-sided and double-sided memory modules in the same system may cause memory related problems (because of timing) and should be avoided if at all possible. Stick to one brand if you can.
Although you can use double sided memory in slot 2 [As you are now] I don't think it will work with an additional single sided stick in slot 3.
You would need to have single sided in both slot 2 & 3 This combination though would work with a double sided stick in slot 1.
Think I've got that right. j.