In case of a Blog, a lot depends upon which topic(s) you write blogs on and whether you're actually interested in those areas or not. Maintaining quality content and regular updates will probably be easier in this case. So, do spend some time thinking about the topics before you really kick-start your Blog. BTW, quality blogging will also be fairly time-consuming.
Starting an e-commerce web site won't be easier either if you target huge success. Blogging is something which is pretty much dependent upon your individual hard work, but success of an e-commerce might require other factors to be well in place. Marketing/Advertisement will probably be tougher for an e-commerce site.
Well... as everyone else is saying here that both have their own pros and cons, but what I think is of utmost importance is what you're actually interested into. Going by that will probably make your life easier. All the best!