Size matters.
Does it make sense to get 5 bids for a new boiler and get 5 different sizes quoted, two of which are the same model and manufacturer? Don't you wonder why? The answer is 4 of heating contractors don't know what they are doing...but which four?
Choosing a new boilers is not about the boiler brand, warranty or popularity, it is about the man that specifies, installs and services the boiler. As it pertains to high efficiency condensing boilers, choosing the proper boiler based on size and features, outweighs all but the installer, whom can often best recognized by his attention to the details that matter.
For instance, the contractor with the smallest boiler is not necessarily taking advantage of you any more than the contractor offering the biggest boiler might be doing you a favor. Much of my work is consulting with people who simply bought the wrong man and are stuck with the boiler de jour. I will say that for residential boiler work the low bidder has almost always made a mistake, either in his job cost or the what the job requires. I would be hard pressed to award any residential work to the low bidder.
I have installed all of the boilers mentioned in this thread with great success, but have also repaired, re-piped, re-programmed and even replaced each of these boilers, depending on the quality of the installation. All condensing boiler are available with extended warranties, so they are of little value, as many problems associated with high efficiency boiler systems do not manifest themselves for 5-10 years, at which point a new boiler or expensive repairs are the only viable options.
It would be great if the average homeowner could pick and choose his own boiler the way he might buy a new car. Unfortunately the experience one gains by owning and riding in many cars is not available to him. By contrast the average heating contractors (dedicated to hydronic heating) has the invaluable resource of a local distributor who works with many contractors, manufacturers and boilers. With a few phone calls your local heating contractor can get current, candid information about any boiler from someone who sells truck loads of boilers every year. For instance one can find out if a lifetime warranty is worth the paper/.pdf it is printed on.
It may also be that a certain brand is well supported in one area by manufacturer's rep, distributor or local contractor but just across the state line no such support/service exists. When I manufactured the first condensing boilers, this was my greatest challenge.
So, to answer your question. Find a contractor that can produce a sample of a proper heat load analysis and perhaps a factory certification from ANY condensing boiler manufacturer. This simple achievement is a good sign that your boiler man is committed to his craft, high efficiency boiler technology and ultimately your safety and comfort.
The aluminum vs stainless steel controversy is silly. Both are serviceable, well proven and well accepted boiler manufacturing technologies, here in and Europe, much like steel and aluminum car engines. Again, the contractor is the key ingredient in any successful high efficiency boiler system installation. Whereas it may be true, that a condensing boiler featuring a stainless steel heat exchanger (there are several common varieties of SS used) is more idiot-proof than the equivalent boiler featuring an aluminum alloy heat exchanger; is this really the standard you seek i.e. can any idiot can install this thing. I assure you they cannot.
http://www.badgerboilerservice.com/boilerchoice.html