Hi smc123456,
Both good choices - I have the GT-4B and are over the moon with it.
The only advice I can give is to think where the stand is going to sit and measure both 100cm and 120cm and see what fits best. It is also worth measuring your TV width wise and deciding whether you want a overhang or not.
I am sure you will be delighted with either - they really are excellent stands and for that sort of money you are having a laugh!