That first motor, which weighs 27 lbs, is grossly out of scale with the Taig; the Z axis motor will have a hard time moving it up. The second one is more reasonable at 5.8 lbs, but since it tops out at 2370 rpm, you'd lose the 10,000 rpm top speed capacity of the Taig, which depends on the stock 3450 rpm motor. Rigidity isn't really the issue here, though - it's important to have a high-speed spindle if you want to use very small cutters..
Is the reason you're going with a servo motor for the spindle to do rigid tapping? Otherwise I don't really see the point. Putting an encoder on the spindle would do the same thing at much lower cost.