First off, I have to say that this is my opinion only and this is just my observation of personal experience. Regardless of what I see, there are no rules or standards in music, and music is just an extension of yourself when you play.....so much of what is said about anything in music has to do with personal tastes and opinions. The best way to find out is to try it for yourself. Regardless of what anyone says, you just might find "Your Tone" is something that's not the norm...
With that said:
I've used both, but I think there's too many other variables that come into play that need to be looked at to determine which is better and in determining better for what. Some of the other variables might be:
Winding technique is used to make the string. (flat vs. round)
What kind of core makes up the string.
What sound are you looking for.
Do you use a pick or play with fingers.
How often do you normally change your strings.
Strenth of the sting depends on how it's made. I've tried both nickle and stainless of different brands that have both broken on me, from the low-E to the high-G. For durability, I've had luck with the DR series strings as well as the Everly Brother brand of strings which are both hand made, stainless steel, round wound strings. Round wound strings are harsher on your fingers but produce a much brighter sound and stay brighter for a longer period of time than flat wound strings. Flat wound strings are often used in Jazz situations for their soft tonal qualities, where as round wound are often seen in funk (slap) and rock styles of music. Depending on the core of the string, nickle strings tend to be more flexible for bending notes where as stainless are generally a little stiffer, but again this is dependant on the string's core. If you don't play much, stainless strings wont rust on you...
So to answer your question, it has been my experience that nickle or stainless are equally as good, but offer different characteristics. The focus I think needs to be spent more on the tone that your looking for and the way the string is constructed.
