I'd go along with the other recommendations as well, certainly the illustrations are by far the best.
However be aware that the Collins guide covers quite a large area
to quote
"All bird species which breed or regularly occur in Europe, North Africa north of 30 N, and Israel, Palestine,Jordan, Lebanon, Syria, Turkey, Armenia and Azerbaijan, that is in a large part of the Middle East. Also included are the Canary islands, Madeira and the Sinai peninsula."
The result of this is that there are 722 species described, which entails that quite often the excellent illustrations and text are sometimes on the small side. Often there are 3 or 4 or more species on one page. The large A4 version of this guide is quite spectacular but very much an at home coffee table book. The two other versions either come as an everyday hardback (non bendable) or paperback version. The hardback opens out flat quite nicely but I find the paperbak because of its binding doesn't open out as well and certainly not in the style of the Sibley with its binding.
The other book I would check out which I think is more field pocketable is
"The Pocket guide to Birds of Britain and north west Europe" by Kightley, Madge and Nurney
http://www.amazon.co.uk/Pocket-Brit...0434846?ie=UTF8&s=books&qid=1173865604&sr=1-1
The illustrations are not up to scratch of the Collins but still quite good and it obviously has a more limited geographical distribution but one that includes both the Netherlands and Austria. It has a total of 384 species which allows it to have clear open text and drawings with usually just 1 or 2 species per page. Its very much in the vogue of the Sibley eastern or western version. It shares with it a bend-able soft/hard back which makes it more durable for field use and also a quick index and is definitely more pocketable than the Collins.
The Collins was designed I think to fit the often prevailing opinion in at least UK birding circles that field guides should be kept at home and you should take notes and look it up at home or in the car.
The Collins is generally thought of as the definive guide to Europe and it is a stunning guide but I think at least for my use (beginner and predominantly UK based) its not ideal and could be a little easier to use.
Comparing the dimensions of all three to the Western Sibley which is what I suppose you are using in Montana then the Collins would be the largest, nearly the same height as the Sibley but defintely wider. The Kightley madge is a little smaller than the Sibley western and about 2/3rds the width, whereas the Collins and Sibley will be nearly equal in width with if anthing the Sibley being thicker which is suggested by the fact that the Sibley has 470 pages, the Collins 395 and the Kightley 296.
I would suggest you get both but the Kightley Madge in my opinion is definitely nearer the Sibley if you're talking about the Eastern and Western field guide versions of the guide.