> OK. So you'd wire them both up in essentially the same way?

Essentially, yes, but you need to bear in mind the differences between the 
two when designing your circuit.  The main difference is that a FET is 
controlled by input voltage, and the BJT by input current.  So in most 
designs with BJTs you'll see some resistors dotted around to control the 
current.

> Is there any major overall difference in operating characteristics? (I 
> seem to recall that whichever one you pick there's about 50,000,000 models 
> to choose from anyway!)

Good point about FET is that they have almost infinite input impedance, 
which is often useful for low-power operation or when measuring very low 
powered signals.  Good point about BJT is that the output resistance is 
higher, so good for driving higher powered devices rather than pure digital 
signals.

> So why is it that (say) a simple NAND gate involves 8 transistors, 3 
> diodes and 25 resistors? That doesn't make any sense to me... Logically it 
> looks like any possible 2-input gate should require 1 or 2 switches and 
> nothing else.

A NAND gate needs 4 FETS and no resistors:

http://en.wikipedia.org/wiki/Image:CMOS_NAND.svg

Inside an IC, resistors are HUGE compared to FETs, there is no way you could 
make a modern CPU if you needed to include resistors for each gate.

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