I'd prefer to buy my friend's vehicle, as I know it has been very well cared for (and it's an easy transaction), but not if I'm making a big sacrifice in terms of towing/convenience/utility.
All three of my trucks have bed rails.
Having hauled for a living their no big deal to me as far as using the bed for other things.
Everyone has their preference...one isn't better than the other. Of course GM may say their system is the best or strongest. Just keep in mind these type products have ratings.
No doubt the friends truck is a heck of a buy and you could look for weeks...months and never find one like it. I would go with a dealer installed system after first looking on that products website for their rating numbers.
