Firstly the type of scarification I do uses a scalpel to either cut a line into the flesh or to remove sections of skin.
The handles I prefer are round dental scalpel handles like the one below. The round body allows me to turn the blade as I go around curves making the job a lot easier and the line a lot smoother
The #11 blade is straight and provides a good indication of the depth of the cut and the point at which the blade is meeting the skin, all helping with the judgement of the depth and location of the cut. The downside is that with tight curves the blade tends to feel like it is catching especially as the sharpness of the edge is reduced over the duration of the scarification session.
The #15 blade is great for small curves but does take extra care to make sure the depth is as expected, also as the blade is smaller doing long lines well can be a challenge.
Also be careful of changing blades during a session, as the cutting progresses the sharpness of the blade slowly decreases, when a new blade is put on it is easy to cut to deep due to the sudden increase in sharpness.