The noob tube requires very little skill to get a kill - just aim at the ground in front of someone and pull the trigger. It is also an extremely CHEAP kill in that it has unlimited effective range.
The RTM is different in that you only get 2 per life (3 if hero). You can't run over an "unused" one in a gun and get more. Also, because they are manually detonated, they can be used in more different ways - not just as exploding frisbees. They can be left as "traps" for on-coming enemies, or even adhered to the backs of teammates, thus making them "running bombs!" Additional skill is required to use RTMs because you are unable to use your gun while deploying and detonating, leaving you open for attack - strategy required.
Also, unlike the noob tube which is available on a preset from day 1, a player actually has to "earn" them by achieving level 35 to unlock them.