I was just having this discussion...
First let me say I love blue guns, there's just something more menacing, more wicked, more manly about blue guns. Of course stainless is convenient and easier to care for. I don't say it's as maintenance free as some would believe, but it is WAY less bother and worry than a blue finish. In practicality/durability nickel falls somewhere between the two. It's more durable than bluing and yet much better looking than stainless. Nickel guns are truly amazing to look at. I don't know anyone that would say they weren't impressed by a nickel finished S&W.
For collecting I think Nickel is the way to go. I believe that today Nickel is way under-appreciated, and therefore undervalued. I (and the guys I was talking with) know nickel guns are more rare than either blue or stainless. Personally I think they are much, much more rare. Take the mod. 19 for instance, there are zillions of these around (well tens of thousands anyway) and I'll bet that not 10 in a hundred were shipped in Nickel. Perhaps as few as 5 in a hundred and certainly not more than 15! I'd also bet the percentage of nickel guns goes way down (and the rarity goes way up) after the corresponding stainless model was introduced. Simply put, Nickel guns were always rare, stainless made them more so. I strongly believe that nickel's rarity will only be increasingly appreciated in the future, and that values will show it.
/b