What is the Difference in Outcome Between a Sill Reduction and an Alar Base Reduction?

That is an excellent question, and highlights the difference between alar flare and the alar base. There are three components when talking about alar base reduction, and these are the nostril sill, the alar insertion, and alar flare. The nostril sill is the flat part of the base, located on either side of the columella. The alar flare is the rounded and/or flared portion of the ala. The alar insertion is where the ala meets the face. Some patients have flared nostrils but a normal alar base, and in this case reducing the alar flare without further narrowing the base is appropriate. Other patients have a wide base but do not have flared nostrils, and in these cases reducing the sill and/or part of the ala is appropriate. Some patients have both flared nostrils and a wide base, and in these patients usually a sill reduction and alar reduction is appropriate. When people talk about an alar base reduction, it can refer to any combination of the above. What is popular is irrelevent, because what would be best for you may not be best for someone else. Also, there are many ways to reduce the alar base, and some techniques leave more noticeable scars than others. It is really important to consult with a board certified plastic or facial plastic surgeon who has experience doing alar base reductions, and ask to see photos of the scars. You can also ask the surgeon to draw on you where the incision would be and what part would be removed. When done poorly, alar base reduction can leave obvious scarring, abnormal appearing nostrils, pinched nostrils, bowling pin deformity, or Q-deformity. These deformities are difficult to fix, so while it is considered a "simple" procedure, it only looks good when meticulous technique is used.

Alar base reduction can reduce nostril size and shape depending on the extent of the scar versus the extent of what needs to be done. Alar base sill reduction reduces the width between the alar bases and this can have a significant change in the width of the nose.

There seems to be some confusion over exactly you want done.  You should see a few different plastic surgeons and have them draw out on your nose exactly what they would do.  This surgery involves artistry and experience and the selection of the right surgeon will be the most important decision you make.  God luck!