If the plans and specifications are properly prepared and the bidders have been qualified by the architect, there should not be a significant difference in the bids. Assuming that this is the case and that you believe any of the bidders would perform professionally and that you feel equally comfortable with all them, it would be reasonable to select the lowest of the bidders.
However, if there is a significant (more than 5%) difference in the bids, this should raise a red flag. In this case, either the plans and specifications were incomplete, resulting in the bidders making differing assumptions that went into their pricing or one or more of the bidders did not fully understand the scope of the project. The architect would need to discuss the bids with the bidders to determine if there were too many assumptions made.
If one of the bidders is significantly lower than the others, the temptation to save significant dollars by selecting him should be resisted. General Contractors are second only to car salesmen in consumer complaints. While it is undeniable that there are a fair number of disreputable contractors, the reality is that most contractors are well intentioned former carpenters who believe they can estimate, perform, oversee and generally attend to the intricacies involved in successfully running a General Contracting company. Often, these “contractors” are able to cut costs by performing their own electrical and plumbing work and assuming the responsibilities of other specialty trades. The result is that the work related to a specific trade is not performed by a craftsman and will invariably take significantly longer to complete.
The vast majority of contractor complaints result from an Owner’s selection of a significantly low bidder. At some point in the construction process, the contractor will realize that he made an estimating mistake and either inundate the Owner with Change Orders or take on another project in order to pay his bills. Since he may have an expectation that the new project will be profitable, his attentions and resources will be focused on the profitable project rather than the underestimated one. The more complex the project, the greater the likelihood that an inexperienced contractor will underestimate the costs and realize at some point that the balance of the contract amount is insufficient to pay the impending bills. This begins a process of unavailability, inactivity and Owner distress. Should the contractor fail to complete the project or be terminated by the Owner, the likelihood of the Owner’s being able to hire a reputable contractor to complete the project is very low.