I think a big part of the S&W 69 in 44 mag, vs GP100 in 44 special issue folks asking "why" miss is that a 44 special in this configuration is meant as a full sized self defense gun, not something to be slinging magnums at an animal you want to eat.
Ruger has the Redhawks, Super Redhawks, and Super Blackhawks for that purpose, and they are far more suited for slinging magnums based on their size and weight.
S&W has the 29, 629, and various X frame revolvers for killing food, which again, are more suitable for that role than the model 69. Based on that, I have to assume the model 69 is meant to be a trail, defense or backup gun.
For self defense against humans, I see no reason a 44 mag is necessary. It seems like using a hammer to swat a fly to me. Will it get the job done? Yes, but it sure seems like overkill. However, on the trail or as backup while hunting, the 44 mag in a small package makes a lot of sense. There are a lot of folks though, who feel the 44 special offers plenty of performance, and enough to handle any threat they may encounter. The biggest predator I have in my neck of the woods is mountain lion, and a 44 special would certainly handle a cat. People and dogs are a bigger issue for me.
So while I like the notion of having magnum capabilities, I don't know that it would serve a purpose for me. I already have a 460 magnum if I want to hunt big game. I don't really NEED a 44 anything for that matter, but "need" became a nonissue for me about 8 guns ago.
IMO, S&W and Ruger are both targeting the defense market with their L frame sized 44 offerings, not the hunting market. Ruger recognizes that a 44 special GP100 will be a well balanced gun for it's intended use, and won't require some of the design features the S&W 69 employs. They also recognize that 44 mags out of an L frame sized gun will not be pleasant or necessarily desirable to a large portion of the market. So they are offering an alternative.
As Denis said, if you want to shoot 44 mags, buy the Smith. If you can appreciate a 44 Special, buy the Ruger. There is room for both in the market.
I am curious to see if Ruger offers a 5 shot GP100 in 44 mag at some point.