Any Rails devs upgrade to a fully loaded iMac 5K? Wondering if it’s worth it to upgrade a 2 year old loaded 16GB SSD Macbook Pro?
Any stats on test runs would be awesome!
I also hear that it’s better to buy the iMac without the full amount of RAM, and then to use after-market RAM. Correct? Does that make sense even with AppleCare?
I hate developing without multiple monitors. That said, the 5K display looks great… Both machines you mention are really overpowered for standard web development, so I doubt it would make you any more/less productive EXCEPT you loose your mobility with the iMac.
As you know, parallelTtests really come down to CORES and RAM (assuming your test DB is small enough to fit in RAM) and HDD TPS (small block read/writes) - so SSD is critical to fast tests, maybe even more than CPU Cores. I don’t have any hard stats to back that up, just a gut feel.
Personally, I never buy new, I have a fully loaded 2008 Mac Pro v8 with 16GB Ram, SSD boot drive, 4 TB usable in a RAID5 and dual monitors that I put together off eBay for $700… With the difference compared to a new iMac 5K, I can bring the family to Maui 
I still can’t wait to get a 5K screen and a faster Mac. My 2012 MBP is a bit over 2 years old. Ancient! And @Kaz, if you like two screens, 3 is even better. My typical arrangement is:
Left Screen (MBP 15", max resolution): Terminal or browser debugger
Middle (Cinema 27"): RubyMine or WebStorm.
Right (Cheap 1080P HDMI 21" screen): Web browser for the app I’m developing.