The SD nav and the tech pack nav are almost identical. They use the same map packs, but the extra processing power of the tech pack allows it to display some additional 3D data.
I've compared both systems, and apart from the screen resolution they are the same.
£500 for the sat nav is, in my opinion, much better value than £1,500 for the tech pack (although this does get you a few extra non-nav goodies too)
The maps seem to be up to date within a year or two. There are some fairly major omissions (hindhead tunnel I believe is missing!), but generally not bad. I will try and check on that postcode for you tomorrow if no-one else beats me to it.