both the GS150 and RP125 would not qualify under any circumstances to be called 'modern'. They're simple and honest motorcycles.
Good torque (very good pulling power even with pillion)
Comfort and stability
Some quality issues
Bad paint quality
Mechanic and parts availability
Ravi RP125 Pros (from what i've heard on the forums)
Some cool and 'modern' tricks like gear indicator, fuel guage
Engine is similar to the honda 125 so mechanics shouldnt be hard to find
Some initial quality issues
The GS150 has a retro shape and the RP125 has a modern look. It's not a modern engineered bike by any standards(both).