Hi Chris,
I have both 'Project' and 'Dee' books and they have both been revised over the years, so are good references. The Project is the simpler od the two and does lend itself to being built into different engines if you have a preference for a particular railway for instance. However, I think the Dee is a better and more modern design to work on even though a little more work may be reqired. It's certainly not going to be twice as much work to have the twin cylinder layout. There's a lot of the work (frames, boiler, platework, wheels etc) that will be pretty much the same whichever loco you build. Both engines have been designed with relative 'beginners' in mind and I know EDee has been built as a "first engine" by many.
From memory, the Dee book does reference some aspects of the Project book (or at least my 1st Edition did) but I also seem to recall that G1MRA sells both books as a package these days (non-Members pay a small increment) so I'd look at their website to check pricing.
The cost of the books will be a small investment given the cost in time and matertials required to build (and hopefully finish) a good engine. Why not get both books and have a good read and then decide what to do. I think you will like 'Dee'.
Regards,
Ian