myopinion: Just go for 128GB SSD as this is plenty to run all your programs and will be a bit cheaper. Store all the big stuff like music, video and pictures on the external drive.
Storing all the big stuff on another drive maybe easier said that done. WIth itunes, it seems to store iphone/ipad apps on the main drive, even though it will store music/video on another. I haven't worked out a way to prevent this from happening yet.