The 6" barrel will have a longer sight radius, but some find them muzzle heavy and harder to hold steady, so there's no telling which you'd be more accurate with until you try them both. I happen to be no more accurate with my 6" K-22 than with my 4" 617. If you can't test fire each, try to at least examine and hold each to see if the 6" feels relatively muzzle heavy to you.
If you're going to shoot it in single action, there's likely little need for 'smithing, as S&Ws typically have excellent single action triggers.
But it's a rare factory gun who's double action trigger can't be improved by a good revolver 'smith, though. You can have S&W tune it, but from what I've seen, you're more likely to get a better action job from a good revolver 'smith. My recommendation would be to shoot it for a bit while doing more research on this before spending your money on an action job.
Keep in mind that rimfires can't be tuned as lightly as their centerfire counterparts without getting into reliability issues. Fortunately, "smooth" is much more important that "light".
