Is the SSD "stick" in the MacBook Pro the same as the Mac Pro?

The model of MacBook Pro will make a large difference as to whether or not it's compatible with the Mac Pro, assuming you're talking about the "new" 2013 model.

Specifically, the Mid-2015 MacBook Pro will work, but earlier generations use various different interfaces which will not be compatible.

As you can see comparing this photo of a 2012 MacBook Pro's SSD and this photo from the 2013 Mac Pro, the interfaces are completely different. Alternatively, you can see the same pinout in the 2015 MacBook Pro on this page.

I recommend sticking with 3rd-party upgrades provided by OWC (since they will tell you exactly what they've been tested with), or only getting parts that could have shipped with the original machine. (For example, ordering a broken Mac Pro off of eBay that came with a larger SSD that you can extract.)

Thats a darn good question. While I cannot tell you definitively what the differences are, the ssds in apple macbook retinas and 2013 mac pro towers are not identical. I think the difference is in the chipset controllers and you may find the retina one does not work in a mac pro, but the mac pro one works in both. Can anyone confirm ?

I do not have a 2013 mac pro here at the moment. But it uses (used) an ahci samsung 941 chip , with the apple pinout. More recent ones may use the 951 ahci chip. The same chips in M2 format work in the PCI slots of my 2010 mac pro tower.

BUT. The Apple SSD from a 2015 retina macbook, with the same pin configuration as the mac pro 2013 chip, does NOT work in my mac pro. It is not recognised in the 2012, 2010, or 2008 mac pro towers. However, it works perfectly in the 4x pcie slot of a mac pro 1,1 (2006 model). As a boot disk, at full speed etc.

If anyone knows the technical answer as to why I would love to hear it !