I remember in Solidworks, when you were in sketch mode within the model, if you were drawing a line, you select the starting point then an ending point, before placing the third end point (or where ever in the line creation string) the cursor was hovered back over the previous point, the line creation would turn into creating an arc. This was so handy while sketching. This prevented the user from constantly having to select between the two commands. This would be handy here as well. I hope this makes sense.