The cons with vents being on the floor is that you have more of a chance on dirt and things collecting in them. We had vents in the floor of our 1st home and our first born would put all kinds of things in it. If you live by yourself, you may not have any problems with that.

We have a HVAC in our attic. Although it is a 2 story, most of our vents are in the ceiling of the 1st floor except for our 2 bedrooms and bath upstairs. They are on the wall. I like this a LOT better. The only problems I have with it is that our return is located above the 4th step of the staircase and is 10' above the floor so you have to get on your tiptoes to access it to change the filter and clean the grate. Also, the builders of the house (we bought it as a foreclosure from the previous owner) "forgot" to put the pipe to the drain from the evaporator. So I have a nice little patch in my living room ceiling for all to see.

The pros to the floor vents is that when you have the heater going, then you'll feel warmth a LOT faster than your ceiling vents since heat rises.

Our air return with the floor vents was in the hall and quite loud, so I can't say how the placement in the crawl space would be.

If you air condition, it is a lot more efficient to have the system in the crawlspace.

I have both floor and attic vents and there really isn't any significant difference to me.