Bolt-ons like smartplant P&ID make an extremely good, simple and productive tool into something extremely complicated and very cumbersome requiring a dedicated team of experienced operators - as different to Smartsketch as you could possibly be.
However, I would avoid the latter by a country mile unless you have an extremely large budget (time AND money), are a glutton for punishment and are forced to use it by others. Smartsketch is the core drafting programme for smartplant P&ID, etc. (AutoCAD is my CAD tool of choice for traditional engineering drafting although there is a big overlap in the 2D). Its has a many novel and very productive features compared with AutoCAD including some that are especially useful for drawing isometrics, P&IDs etc. Its not free but IMO its the best tool for the job and one of the best 2D drafting programmes for both engineering drawing and non-scale 'sketching' such as isometrics and P&IDs. The best CAD software for 'manual' (rather than automated) drawing of isometrics is Smartsketch.
