it's worth noting that if your house doesn't include wall insulation in external walls, you should definately add it if possible.
thicker is better, but the thickness of the batts should match the stud depth (typ. 90mm) -- you could squash thicker batts in but they won't achieve any better performance, as it's the air inside is what's doing the work.
R3.0 is 115mm thick (so no good for normal stud walls), whereas R2.0 is 90mm thick.