It then becomes a price/performance comparison. Large dimms are more expensive (and a little slower) but you can fit more RAM. Large and cheap dimms are much slower and by much it is in ms so will you notice it? Maybe not, yes if benching.
2 x 4 will give you more stability, 4 x 2 will give you lower timing (4-4-4-10, 4-4-4-12, 5-5-5-15)
Also have a look around how your chosen motherboard handles any RAM brand/config you consider.