L115 is better overall, IMO. Largely due to the fact that it has a slightly bigger OHK zone (knees up, as opposed to the USR with the waist up).
Personally, I don't use Lethals or Tacticals.
I tend to use the Variable Zoom or Thermal Scope. For a second attachment, either the Chrome Barrel or Extended Mags, though I tend to go with Extended Mags given the Chrome Barrel's obnoxious extra kick (and being a bolt-action rifle already, if you miss your shot, you more than likely won't get a second chance) and the extra damage isn't really necessary.
P226 with the Muzzle Break or Tactical Knife is usually an all-around good choice.
Support Killstreaks are usually preferable to me, given you probably won't be racking up many kills very quickly (and snipers are usually meant for a supportive role anyway) unless you're just THAT good. What Perks and streaks to use are up to you.
Honestly it really depends on your playstyle. If you're looking to be more of a passive sniper I would recommend the L115. With the L115 you have a smaller chance of getting hitmarkers but the L115 is a slower and less mobile sniper rifle than the USR. If you're always quickscoping and playing aggressively then the USR is definelty the way to go. Personally,I use the L115. My main setup is below.
I also don't use Chrome-lined because I don't like the extra sway it gives me. I like to have as little sway as possible.
I prefer the USR because of the speed over the L115. I usually play aggressively and the Quickdraw perk helps out with the ADS speed of the USR.