While 128gb will be adequate for your needs, consider this:
The difference between the 256gb ($1399) and 128gb ($1289) is $110. If you factor in the price of an external HDD USB drive, which is roughly $50, you are looking at a net difference of $60. Even a USB3 external HDD will never approach the speed of the internal flash disk. Running Parallels from an external HDD will be less than ideal. I've done it and I prefer not to do it. With an external USB3 SSD, the performance is acceptable but this is not a cheaper alternative than going for the 256gb in the first place.
So, for the price difference, I wouldn't even think twice and go for the 256gb. That's my 2¢.
You said that very well, and I agree