This is a difficult question to answer as it depends on many factors. As a general rule of thumb don't just select a store on the basis of the lowest price. Often the cheapest suppliers are very unreliable and carry little or no stock. Try to choose suppliers with good reputations - saving $5 on an order is of little consolation if your order takes two months to arrive. I strongly recommend checking the reputation of your chosen online store at the following sites before placing your order.
Another good idea is to do a google search using the companies name. Keep in mind that many bad companies pay people to write good reviews in sites like resellerratings.com. Don't just rely on the overall score for a store, but read the reviews. In particular, watch out for reviews that describe the store using a "bait-and-switch". These scams seems to be most common in certain product categories (such as photographic equipment), while in other categories these scams never seem to occur (e.g. clothing and shoes). Since I check all the orders I will pick up a lot of these scam sites, but it is always best to make sure yourself. The most important question to ask is would you be happy to order from this store yourself if they would ship directly. If the answer is no then you should be very careful about selecting this store.