We tried hard to tile our windowsill, but had the same problem as you trying to figure out how they should look. We thought of using chair rail tile at the edge, but it seems to create a number of installation challenges, as well as running up the tile cost (corner chair rail tiles are not cheap).

In the end, we went with a slate slab for the sill. This has been totally fantastic for us - unlike the old wooden sill, it doesn't mind when we put a flowerpot or a wet sponge on it, and it looks great. Moreover, you can probably get a remnant for this, so the material cost is very reasonable (I think ours was $100 for a 6" x 50"ish slab that was trimmed to fit).

One thing about this is it also gives you the option to have a deeper sill if you want to do something like put flowerpots on it etc.

It worked out great for us, so just another approach you may want to consider.