I've got both models. I bought a D100 in July 2002 and carried my older film F100 as a spare body, but the fact is that I only made three or four rolls in two years since the D100 purchase, so I decided to sold it and bought a D70 in exchange (by adding less than 100 Euros).
I was wrong: the D70 couldn't be my second camera, it's now my first body and 90% of my photos are made with it, specially action shots (with the 80-400VR attached). I use the D100 for ramp shots with the 28-105 or the 18-35.
Here are the D70 advantages for me:
- D70's bigger buffer allows me more shots in a sequence (I shot mainly RAW)
- Metering is much better. The D100 tends to underexpose, specially in low light conditions (Though you always can adjust exposure or shot manual, like in old times
- Compressed RAW (half the size of a D100 NEF file!)
- Better LCD
and zoom display
- Focus confirmation by a beep
- You don't need an AC
adaptor to clean the sensor (even cheaper
The D100 also has its pros:
- Better build quality
- Better noise performance (at least to my eye)
- Two memory banks for your preferred settings
- Wider ISO range
Anyway the D100 is an excellent performer, but if you are money conscious go for the D70...
There is a very fine line between 'hobby' and 'mental illness'.