[Joe]

For my money, and if I had to choose between the two, with main use being to fit to any high magnification lens set up, there would only be one choice..
The Pentax K100D.
Slight downside is it's use of AA batteries...and it does come with some rather cr@ppy alkalines. I thoroughly recommend the use of two rechargeable CRV3 batteries to overcome this.
However, MAJOR upside is that it has a body intergrated anti-shake feature. Yes, it isn't the best form of image stabilisation in the world. BUT, it's better than having none at all ( as with the Nikon D40).....an important feature with your application I feel.

Remember also, if you want to extend your lens range at a later stage, the Nikon D40 uses only AF-S lenses (if you want autofocus).....which, quite frankly is a pain in the butt, as most Nikon lenses to date (other than expensive VR galss) are AF only....as are all the independants e.g Sigma and Tamron
By contrast, there's loads of lens options available for the Pentax, which will retain autofocus and metering etc.