Costco and Dollar Tree are performing well in a challenging economy, but one has a stronger track record of success.Consumers are looking for bargains today. That fact is highlighted by Dollar Tree (DLTR +1.02%), which is seeing more and more shoppers from higher income brackets. However, Costco (COST 2.70%) is also a leader in providing products at attractive prices. Which one is the better investment right now?Very different business modelsCostco is a club store, so consumers pay a yearly membership fee f ...