Which one would be better, if we look it @security side?
I heard that RDP is very very poor when it comes to security.
RDP has some issues, but these are broadly mitigated by getting the config right. Wicked Clown demonstrated a wonderful privilege escalation attack from restricted user to admin in less than 5 minutes at the recent B Sides London security conference which relied on a common misconfiguration - paper here. TLS can provide a strongly authenticated tunnel - which will help.
An alternative could be VNC - which also has potential issues, however using an SSH tunnel for this gives you a much better position with respect to security as you can use strong mutual authentication.
The key point to look at is what your key risks are - if you are worried about an attacker pretending to be a user, SSH or TLS with certificates gives you the authentication and encrypted tunnel part of the puzzle, is a good idea, however you still have the risk of an attacker stealing the mobile device:-)