If you've settled on a S&W, then the basic choice is between an N frame 27 and an L frame 586/686. They balance very differently and have different trigger reach and "grip" sizes. The 586 & 686 have a ton of weight out toward the muzzel which rules out comfortable one hand use but if you're planning on firing laying on your back with the barrel rested across your ankle excess muzzel weight is not a draw back. The 27 is well ballanced and comfortable to shoot one or two handed offhand. It's larger "grip" and longer trigger reach make it harder for most folks to shoot DA but its trigger reach in the SA mode you're more likely to use is only a problem for the small handed. 27s were not made with adjustable front sights like the 586 & 686 were. There also isn't a stainless equivalent to an 8 3/8" 27.

If I was planning a purchase to use in a standardized match I would go to a match first to observe what others are using and ask them for advise. In the worst case you could show up at your first match with a gun that is not allowed due to some odd ball rule you never heard of.