It realli is a matter of personal taste. Some people, including myself, quite like the G12-75ts you have in you amp at the moment.
They are all Eqd differently, and you have to decide what suits you.
The vintage 30s are extremely loud, and have an upper-mid peak which means they cut through very well live. I personally dont like the upper-mid peak, as i feel they sound slightly harsh, especially on cleans (this is a peronal opinion!).
The Greenbacks are not nearly as powerful, and break up much easier. They have a woodier and slightly warmer tone
An excellent compromise between the two are the G12H speakers, which have the power of the Vintage 30s and not the upper mid peak, and are a bit richer and warmer (more low-mids)- very 70s.
THe G1275s which you have have a much less pronouned midrange than the others.
An increasingly popular thing to do, which when i have heard it works really well, is to change two of the 4 speakers in a 4x12, in a diagonal pattern (ie speaker type 1 in bottom left and top right position, and speaker type 2 in the bottom right and top left).
Ao if you kept two G1275s and changed 2 others to either Vintage 30s or G12Hs depending on taste, you would get a huge sound with excellent balance across frequencies.
If you havent already, go to
www.professional.celestion.com and listen to the samples. They are not great, but give some idea.