There is a definitely a divide between working line Mals and bench bred (show line) Mals, for sure. I would say a similar divide between the bench bred and working border collies (maybe a little less extreme, though maybe just as much, depending on perspective), maybe even similar to some of the bench/working gun dogs, like Goldens, Labs or Cockers. The main thing is that confirmation is only important so far as it allows the dog to do their job well, their can be a much wider range in appearance and size depending on line and specific purpose.
I have heard that show line Mals tend to be more nerve-y, and they don't have the same drive that working line dogs have. That said, this is from lurking on a hardcore working dog forum, so I take it with a grain of salt. I definitely believe they don't have the same drive as working line dogs, but don't know about the nerve aspect. I will say, I was not super impressed with the Mals I saw in the breed ring at Westminster when I was there, and without fail the Mals I think "wow how fun!" are working line and not bench/show bred. The bench line kennels are just valuing something different, IMO, and that something has a lot to do with looks, which to me is not something that should be the focus of a Mal breeding program. I do also tend to have a bias against bench line dogs and show bred dogs in general, though- I will always prefer a health tested hobby breeder who competes in some kind of sport, doe therapy work, or something like that with the dogs, or a working line, if those exist in the breed.
I have seen it suggested if you're looking for just a pet, a show line dog might be preferable. Even for a non-bitework-sport, like agility, a show line dog could be an OK fit. If you're getting a dog for bite work (sport, protection) or police work, a working line Mal is what you want. A working line dog is definitely not the go-to choice if you're just looking for a pet, unless you're looking for a really intense pet and dog training is your hobby and you are very clear with your breeder as to what you're looking for. Even within the working lines, their is distinct lines that produce very different dogs- some are KPNV (police force) bred, some are bred for schutzhund, some for french ring, some for the newer sport of mondioring, some are mixed lines with a bit of everything in it. I'm still new enough to the breed I haven't fully sorted the differences in the lines, though, LOL. It is really fascinating stuff, though. I am curious as to how true the nerviness issues in show lines holds, though.