There is really not much comparison between the two. Both tools are similar only in that fact they both hold a router bit and can rotate it to cut material at high speeds. A router is a 110V consumer grade tool that is rated for intermittent use while a spindle is an industrial motor that has been manufactured to run all day long, day in and day out for exactly the purpose you want to purchase it for - CNC cutting. Spindles are more powerful (note that hp ratings between routers and spindles are not comparable) and will maintain their full torque down to much lower RPMs than a router. Spindles have precision bearings, which means less run-out and thus smoother and more accurate cutting. The bearings on a spindle also stand up to continuous use better than those of a router. Bearings in a heavily used router will need to be replaced every three-four months, while a similarly used spindle should be good for a few years between bearing replacements. A spindle is also much quieter in operation than a router.
That been said, if you don't need a heavy cut and noise is not an issue, then a router is a safe choice. In production situations, we highly recommend a spindle. In addition, the full performance capabilities of our machines are fully realized with a spindle because a router will bog down in heavy cutting at higher cutting speeds.
In making a decision, the real dividing line between the two is on how you make your money. If your paycheck comes from someplace outside your shop, then buy whichever you feel you can afford. If your paycheck is generated inside your shop, and your living and reputation is on the line, you need the security and substantially higher cut quality that comes from a spindle.