They are both made in the US, small, owner-operated brands. They both make super high-quality stuff that's built to last and punches above its weight for cost-to-value. All things being equal, you'll need to decide on what cut will fit you best, and what you're looking for in terms of denim.
This is subjective. RT does have more styles, but that doesn't make them the better choice. RT has a couple more fits than 3sixteen, so that may help. If you bring 3sixteen+ cuts into the mix, there's some great options there too.
The ISC fabric is very similar to the 3sixteen Shadow Selvedge, though the RK cut is unique to RT.