If I had to choose just one thing I’d go with 8GB RAM. There’s no way to upgrade memory later so if you buy the stock 4GB RAM model and need more RAM later your only choice is to buy a new computer. Even if you find the 128GB SSD too small later you have alternatives including moving less important files to an external hard drive (music, old projects, and video are good examples). You can also purchase a SD RAM card for the SD slot to store additional files on - I have a flush mount 128GB SD card in mine which is where I store my music and older files. And while it will be expensive you can be fairly certain that eventually an upgrade SSD will become available.
If possible I’d recommend both the 8GB RAM upgrade and the 256GB SSD but as I said, the RAM upgrade really is a must. BTW, I have a huge amount of software on my computer: Creative Suite, Office, and tons more and have no trouble keeping it all within 128GB. What did push me over to make the larger drive a must have is that I also need to run Windows so I have 45GB tied up in Windows and Windows software. (I’m a programming instructor and need to test my assignments for Win and Mac users.)