Not sure what you plan to do with the one you choose, but here is my take: I have owned them both and...personally, in regards to a 357 magnum I like the smaller frame and fit of the L/686+. It carries nicer, fits my hand better and to me just has a better feel when I shoot it. A good comparison would be hunting or carrying around a 20 gauge shotgun that is built on a 12 gauge frame like the older 870 Wingmasters.
All that said, if you are going to carry and shoot a larger frame gun then why not go with the larger frame caliber???? This is a question I had to ask myself and so I tried a 629 Classic and fell in love. I should add that I use it to carry in bear country and that was my primary reasoning...but after spending most of my life not interested in 44 magnum caliber I am very glad I got interested because I really like it!!!! It seemed like every time I had the opportunity to shoot one it was owned by some idiot that had the big cowboy hat with feathers down the back, a big 10 inch in diameter silver buckle, pointy boots and a loud mouth. Telling me that "sooner or later that 357 will grow up and be a real gun, a 44 magnum!!" He reloaded and always went 10% over maximum to impress everyone at the range. I couldn't hit anything with it like that and so I veered away from the caliber.
I finally met up with a normal guy that had one and tried his and have been impressed ever since. I would suggest you consider the same.
On the subject of barrel length...if you are hunting or may have to someday shoot at something that might be a little long, the longer barrel gives a better sight radius and makes it a little easier to hit the target. Also, I like the full under lug it looks cool and helps keep the muzzle down a bit.