Re: The RedHat corporate spinners won't fix this
90% of the difference on a server is the package manager (yum vs apt), the rest is rather minor. A few differences in default packages, and a few different configuration helper programs. Maybe also a few differences in how they structure larger service configurations, like the nginx folder.
Switching difficulty is down to how you use it. If you're just using e.g. a web server with PHP, it's pretty simple, especially if you have no special configurations. Just install the new server and copy over your stuff (this is definitely the best way for cloud services, if you're looking to do auto instantiations, where setting up a new server must be scripted).
Perhaps unsurprisingly, the more complicated your setup and customizations, the more trouble changing is, and the more you will realise the differences.