Rex 1 is the faster of the two and better for singles, but if you plan to shoot reclaimed shot with it I'd recommend dropping the powder charge below what you would normally use. If it was me I'd start around 16.5 gr for a one ounce load depending on your primer and wad. Check your pattern and be sure to weigh the shot charge dropped; reclaimed is famous for light weight drops. Good first shot at doubles load; for the second shot I'd use Rex 2 WITHOUT the reclaimed shot!
Use the Rex 1 for your singles as it is a little faster than the Rex II. I have done some serious testing on the Rex powder and although it's a little dirtier than the premium powders, it's very consistent in both pressures and velocities. Use your Rex II and III for handicap and field loads.