I had used the SL24 for about three years and then I purchased a A&H QU 16 and a Midas M32R (which at the heart of it is the X32). And since you mentioned the Rack X32, I own and use a QU-PAC for mixing headphones in our home project studio.
Hands down the X32 versus the second generation SL is no contest. X32 is the winner. But, when it comes to the newest generation of SL I have no idea, I have not seen or used the new SL board, but I bet Presonus has done a hell of a good job on it. I will say that when I did own the Presonus and had a surge take it out, that having it repaired through the manufacturer was really easy and economical. Also transitioning from analog to digital may be a little easier on the SL. For the technologically challenged the second generation Presonus would definitely be an easier transition in my opinion.
If he is considering the rack mount I would look at the A&H QU Pac. I feel that the QU has a little easier control using the touch screen. I considered the X32 rack because it was a little cheaper, but I have not regretted the QU_Pac. and it is expandable using the digital snake just like the X32. Both are really meant to be installed in a rack and controlled from a tablet, but I still want to be able to easily use the console if I loose network or the Ipad crapped out and the QU-Pac seems to have a better onboard interface for controlling the console. I have also used the Ipad control app for the Midas and the QU series and the QU series is easier to learn, even though both are great apps.