Since I'm installing both operating systems for fun I decided to compare them. Unfortunately I did not get past partitioning the drives.
The first thing I noticed and forgotten about was that Slackware requires that the administrator partition the disk system manually. The installer reads the partition tables looks at the partition types and then decides the installation strategy. By comparison the open BSD system gives you an opportunity to install the operating system using their opinionated partition table. I have installed many hundreds of computers over the years and therefore partitioning drives is not something that's unfamiliar to me however I have gotten used to be openBSD installer.
I have also gotten used to the fact that not all of the software that I depend on in a minute some environment is available on the openBSD.