I just read Kris Buytaert's blog post "Honest Open Source", which talks about how customers can trust open source consultants to recommend suitable software because, unlike commercial software, there are no incentives for selling licenses. His point balances on the fact that most open source products do not have partner programs to pay consultants when they implement the solution. While some open source companies use other methods of encouraging their partners to recommend their products, the business influence over technical decisions is certainly less than in commercial software. You may remember the case of Accenture taking kick-backs while having the Department of Justice pay them for a "Vendor Neutral" product selection. With millions of dollars of licensing revenue at stake, scruples are going to be tested.
Still, even without kick-backs, people have their biases. Developers like to use software that they know. Working with known technologies improves efficiency and reduces risk. When you are doing a fixed bid proposal, this is very important. I pretty much beat this point to death here. Most of the open source consultants that I know are affiliated with a project on both a business and an emotional level. At a business level, most of their work is associated with the project. On an emotional level, they believe in the community and the software. They are usually pretty candid about their connection and position their opinions accordingly but sometimes the passion gets in the way. You could say, bias through passion is more honest than bias through reward but to the customer it is still bias.
My advice is to talk to different companies and accept their partiality. Have them defend their opinions and disclose their affiliations. I find that talking to the consultants that will be staffed on the project is the best way to get a straight answer. They know better than to over-promise on the technology platform because they are going to be around when the truth is revealed. It is going to be their fault if implementing a feature turns out to be difficult and they will be left holding the bag.
 Readers of Open Source Web Content Management in Java know about these strategies.