Please recommend a good technical design tool for iPad or OS X

I'm looking for a tool that will allow me to create isometric and flattened designs. I'm looking for an application that allows designing of engineering projects for wood working.

I've spent a lot of money looking for an app but most are tools for sketching and artwork orientated.

Some features I'm really looking for are:

• Easy to use
• Isometric and flat views for same design to be shown
• Vector based design, allowing me to resize, move objects individually
• Measurement and scale support, allowing me to specify a scale and then size elements based on that scale (in metric notation)
• Layers support would be preferred to allow measurements to be supplied on a separate layer

I don't want an overly complex tool, I just want a nice tool to do the job of creating designs alleviating the need for me to use pencil and paper.

If you haven't already, check out Google Sketchup. I don't think its on iOS yet, but it is lovely in its simplicity, short learning curve and flexibility. There are tons of YouTube clips showing off features and tutorials.