The MacBook will charge the iPad at full speed, due to Apple increasing the power output for their own devices. You should get the full 2.1A, same as the iPad charger.
See:
http://support.apple.com/kb/HT4049. It should tell you how much power you are getting.
The desktop will not have this feature, so it will charge very slowly (0.5A). The iPad will say its not charging, as the screen will need more than 0.5A to stay on. If you still want to charge it, it can take up to 24 hours to charge this way, keeping the screen off (I did this once as an experiment). Turning on Airplane mode will shorten the time, but not by much.
Devices request the power they need over USB, so it will be safe to charge (the charging power will not exceed any limits). The only potential issue is if you use a cheap charger, it can give the wrong power amount and there were news reports of a bad charger electrocuting people.