Sufficient ssd for Mac This is my first query , and also my first apple product, macbook pro retina.

Should I be buying 128 or 256 as I am really tight on budget.

I have 2 tb of external but I dont know I could make use of it.
I do some programming on Java and c and also python.

Thanks in advance
128Gb is plenty, with the condition it it is only plenty if you do not have much media (pictures, music, video). I have set up plenty of company laptops with 128Gb SSD drives, none are in any danger of running out of space.

If you have media, and can offload it to an external drive , then you are still fine. iTunes and Photos both allow the media library to go anywhere you want.