AFAIK it amounts to the same thing. One has more 2X the number of chips but HALF the memory on each chip.
If you can't get the EXACT same model number (i.e. to go from 4GB to 8GB), then make sure the new memory:
a) is rated for the same FREQUENCY
b) has the same CAS rating
For example, DDR3 1600MHz CAS9 (and obviously is the proper memory TYPE, SODIMM or whatever for your laptop)