I would expect it to be the latter...
In my experience it depends on several factors...gender, muscle type, definition of "easier", level of conditioning, preferences and amount of time one can spend resistance training. Personally, I have found it easier to achieve toning but I am a 6' tall woman.
For me, it has been easier to achieve muscle toning. Being a female, over 40 I am happy to not be losing muscle mass. My brother was an avid bodybuilder and my experience from watching him through the years was that it was more difficult to increase muscle mass than to tone you muscles. He was about 5'11" and went through very regimented workouts most of his life. He was very muscular and toned, but never got to the level that you see in the muscle magazines. He had the build of the guys you see in the bowflex commercials. He also only used protein powder type supplements and no drugs. Anyway, that is my 2 cents.
This depends on a lot of variables. For people who have high metabolisms gaining even a pound of muscle mass can be a huge task. For other such as myself I can put on muscle mass but its very hard for me lean up and get toned. All you can do is tailor your approach to what you are trying to achieve. I firmly believe is this:
For muscle toning - high reps (12-24) with lower weight
For increasing mass - lower reps (2-12) with more weight
There are a million opinions out there but this one seems to be pretty universal in the fitness community.
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