"Does a wireless pci card on the mother board perform better than a wireless usb adapter?"
No, not in my experience.
"Should I expect a degraded connection changing from cable to wireless?"
Yes, definitely, and more prone to connection being lost intermittently.
If your router and PC are close enough for a wired connection, it's absolutely pointless going wireless just for the sake of it.
"Trying to save some bucks".
You will, by not buying a wireless adapter unnecessarily.