I want a smoother cut than I will get by scoring and breaking. Would I use an abrasive wheel in a circular saw or a diamond blade?
a diamond blade is good yes, but running it perfectly straight is the difficult part. Make your self a straight edge so your saw or angle grinder can rest against it as your cutting ( say the blade is 50mm from the saw base or angle grinder guard, put your straight edge 50mm away from your line) when it is cut get a piece of cement board and rub it along your cut to achieve a factory finish you could also buy a cement board blad ( it has only 4 teeth and it can fit onto your circular saw,) other then that you can use an angle grinder with a diamond blade.
Masonry blade on a circular saw would do it. Clamp a board on the cement board and run the saw along it. take it slow and use several passes. You should get a fairly smooth/straight line.
