If you haven't bought your system yet I would get the bigger straight SSD model (sans the HD). Getting the needed SATA cable is not hard. But getting the custom Apple SSD is quite difficult! Get the largest you can afford. Then for now just get an external Thunderbolt or USB HDD drive.
In a year when the warranty has expired think about upgrading the system with a second internal drive. By that time the SATA SSD options will be cheaper.
Opening up the newer 'Thin Series' is a bear and if you're not careful its quite easy to damage the display which would be a very expensive Opps! Besides Apple won't honor any warranty repairs (first year) if you've opened the system.