mini mill spindle vs trim router

I'm would like to build my first minimill. I have three acme thread
driven cast-iron positioning tables for the x, y, and z axis and two
6 inch rotary tables for the 4th and 5th axis. The question I have
is which would be better to use for a spindle. Would a 3/4 hp router
that can turn at 30k rpm or a .5 mini mill spindle with variable
speed control that runs 2500 rpm be better? Here's the spindle
http://www.littlemachineshop.com/products/product_view.php?
ProductID=1912
It says it can drill up to 1/2" and face up to 1.25" and end mill up
to 5/8". Has anyone compared these? I'd like to mainly do small
cuts engraving and the like in aluminum.
How do the spindles compare for run out? Is the effect negligible?
Thanks in advance
Virgil