Unfortunately you can't upgrade a MacBook Pro Retina in any way. Thus you need to max it out if you are a power user. Mine listed for $3199 in Dec 2013 with 1TB SSD, 16GB RAM, high end CPU, Graphics etc. All soldered to the motherboard (forever).
I didn't realize it, but the 1TB, $700 SSD drive hard soldered to the motherboard inside my 3 year old MacBook Pro 15 wRetina Display was bad (or the controller is bad). How do I know? My MacBook Pro crashed 10 times a month -- through 4 operating system upgrades and several rebuilds. Nothing worked.
Then I bought a Samsung T3 SSD and started running my macos Sierra in tethered mode (my main drive is connected via USB). In 2 months there have been 0 crashes. The Samsung T3 saved my $3200 MacBook pro from being a brick..
Sure.. Being tethered is like a ball & chain -- but hey.. the "ball" weighs nothing (and the cable is under no strain).
In any case.. give the Samsung T3 SSD a look. The speed is amazing (about 400MB/sec compared to 30MB/sec for a typical 5400 external drive). It's about $180 per 500GB. The prices are linear -- a 2TB is the price of two 1TB drives and 4 times the price of a 500GB. The Samsung T3 drive also features hardware encryption (much faster than FileVault 2 which uses your CPU and RAM to encrypt decrypt data to/from a drive).