Encoders

I'm looking to get some encoders for my mill conversion. I have .200"
lead ballscrews - it looks like my reasonable choices are 500cpr, which
gives me 0.0004"/0.0001" resolution (w/quadrature), or 1000cpr which
gives me 0.0002"/0.00005" resolution.

If my calculations are correct, with 1000cpr w/quadrature, I should be
able to get a maximum of 135ipm rapids using Mach2 (45,000hz pulse stream).

Assuming that 135ipm is satisfactory, is there any reason NOT to use
1000cpr encoders?

Is it possible to switch quadrature on/off on the fly? Or even without
too much trouble manually? It might be interesting to have a machine
setup with high and low resolution depending on what you were working on.

I'm currently looking at US Digital encoders - any other recommendations?

Thanks,

--C.S.