Depends on what machine you are talking about, having a big external SSD driven through a USB2 interface won't be very great.
I have onboard 120GB SSD and 1TB HDD, works great with media on the HDD, sizing drives needs to be done by your space requirements...
I'll be using my Mbp as my portable entertainment machine along with my iPad. I won't be installing intensive apps on it as I have a powerful separate desktop for that. So mainly media - music, films, photos etc.
Any idea on volumes? For instance I run the 1TB HDD as I want (not need) to carry around my photo catalogue (600GB), iTunes media (120GB), and have a backup bootable OS partition on the HDD (120GB), and have some growth space. SSD is OS and Apps.
My desktop has used up around 250gb and that includes the Windows OS, all my music, photos, movies plus a few professional apps and games.
For media you can use cheap USB sticks or SD cards (preferably small ones that don't stick out much like the Sandisk Ultra Fit or Transcend Jet SD card). They are much slower than SSDs, but easily fast enough to store media for playback.I'll be using my Mbp as my portable entertainment machine along with my iPad. I won't be installing intensive apps on it as I have a powerful separate desktop for that. So mainly media - music, films, photos etc.
I have a MBP for portability. Having to carry around an external drive to use it for normal tasks defeats that portability. I want a drive that holds the files I commonly need. At home I have lots of network storage available which is also where I use most media files. If I want some when traveling, I can copy those over and delete them when done.
To paraphrase, you can never have too much RAM, disk space, or bandwidth.
What are the pros and cons of having a 128gb internal SSD with external SSD vs having a 256 or even 512gb internal SSD?
How much internal storage do you have?