I'v spent a lot of time on a KX100, several 125's and 250's. The 125 will feel a lot like your 100, but will pull a little stronger and have a much more stable chassie and better suspension. I don't know anything about green stickers. A 125 makes its power in a very similar way to the 100, revs high and quick with a fun hit in the mid-top. A 250 is much different, it will make decent power down low, great power in the mid-range and sign off early compared to a small bore. Fun, but very different. I think you would really like the 125, its the best way to get used to a full size bike. Unless you are riding sand dunes, you are not likely to need any more power than a 125 will make. Lastly, riding a small bore teaches you to be a much better rider as you will learn to keep the engine in the sweet spot, keep up momentum etc, where as a 250 will tend to make you lazy if you transistion too early.
good luck and have fun:)