If people are willing to pay for the download and they want it on paper, they will likely be okay with shelling out about £1 of printer supplies for the print.

I know I will :D

Larger margins in a printed documentalways make me happy—they provide space for marginalia.  It does look a bit weird when you flip a page and the wider margin changes sides, but I typically print large documents in 2-up format to save trees.

1. A PDF download was promised; stick to your word

2. Sales of the print product will be a tiny subset of the downloadable product. Downloads are more ‘accessible’ to those of us who live in parts of the world that make posting kilograms of paper prohibitive.

3. You’ll make more money off a downloadable product (if 37signal’s experience is anything to go by)

I would suggest that you hold off on the print version for a few months, because:

1. Early readers are going to give you some great feedback that you’ll want to incorporate in to an updated publication. Better to get the errors fixed and the updates done before you commit them to the more permanent print medium.

2. If the downloadable version is successful, publishers may want to take up the print project with you: way better quality than Lulu.

That said, the downloadable and print versions need to be treated differently. The title of your book seems to assume that each medium requires it’s own design treatment … you’ll get the most out of each medium by designing to it’s strengths.