Sigs are nice... so many choices, options. Suggest you search the AR15 forums for experience from owners, and search Chuck Hawks - a good resource for handgun and rifle reviews.
I have 3 Bushmasters, 2 of which are older LE carbine trade-ins that were produced in the ME plant, which is now run by Windham Weaponry (former owners of Bushmaster, and same tooling). New production is in Ilion, NY - traditional home of Remington Arms. The Bushmaster XM-15s are well built, to say the least. I don't care for their new polymer "Carbon 15."
Since you are looking at Ruger, take a look at their Mini-14 "Ranch Rifle." It's modeled after the famous M1/M14 platform. Not as customizable as an AR-15, but they are great fun, and have lots of variants - blued, stainless, wood stocks, composite stocks, etc. Earlier models were said to have accuracy problems, but Ruger made manufacturing changes in the mid-2000's to improve that (heavier barrel near the front handguard is the easy way to visually tell the updated model). If you get one, suggest you get it chambered in 5.56 NATO, not .223 (Ruger makes both chamberings). If the rifle is chambered in 5.56, you can shoot either that or .223. If it's chambered in .223, you shouldn't shoot 5.56 due to likelihood of higher chamber pressures than designed-for.
Have fun!