I have a choice between two motors:
but the torque of the one from maxonmotor is 0.5 NM with the gearbox,
while the one from tetrix is rated at 300oz per inch
Could somebody please explain why are they different and by how much , which would be better :-) ....
The maxon combo has a stall torque of 0.216 Nm, I don't know how you arrived at 0.5 Nm?
The 300 oz/in (Tetrix) is 2.1 Nm, but... you can bet the blue of the sky that it's the stall torque and the max. efficiency is at somewhere around a fifth of that tops (i.e. 0.4 Nm) and the 152 RPM is the unloaded speed.
Assuming the Maxon goes 6500 RPM (it won't go that low, but just to get the point across) when loaded only by a gearbox. The reduction to get to the same 152 RPM would then be: 6500/152 = 42.76
The stall torque of the Maxon is 56.9 mNm = 0.0569 Nm.
After the proper reduction it will be 42.76 x 0.0569 = 2.4 Nm.
So, reduced to the same unloaded speed on the output shaft, the Maxon, even when being pessimistic, is stronger, but only by 14.3%- not extraordinary consider their relative price.
Better or not depends on several factors. For once, you can't get a 42.76:1 gearbox, The Maxon solution seems to net at close to $200,- while the Tetrix is $30, so you could get 6 of the latter and have change to spare.
Better is the one that suits your 'bot, as it will stand when finished and loaded and comes with the lowest price tag if two or more motors are very similar.
As a rule of thumb: If you get that much specs that the Maxon comes with, it is a good motor. The Tetrix is hopelessly underspec'd, although it might still be what you need, but you go about it the wrong way - first you gather all the data of the finished vehicle and then you calculate the motor specs. you need and if you wanna do it absolutely right, you find a motor that will accomplish that with a nice reserve.
My robot is driven with omni directional wheels three of them :
they would be positioned 120 degrees from each other.
and my robots weight is 2kg not counting the motors.
Could someone tell how much rpm my motors would do with these specs
No, it needs quite a bit more data on each motor as well as on the rest of your setup, but that is still the wrong way 'round.
Set up the specs and then find a motor that complies with that.