What are the points cost for each? They are different hotels in different areas although only about a mile away from each other. You could walk from one to the other easily.
I lived near the Shelburne, so I am intimate with Murray Hill. It is more residential. You'll have quicker access to public transportation (Grand Central Station), closer to supermarkets, cheaper parking lots etc. (not that I would drive).
The Regency is more upscale in a higher end business/tourist area. You'll be closer to the Madison Ave shops, Central Park Zoo, Carnegie, FAO Schwartz and you still have good access to public transportation.
Again let me say, they are within minutes of each other. If I were going to explore Manhattan, I'd rather be close to 42nd Street. If I were going to spend more time in the hotel and walking nearby I'd opt for 61st Street and Park Ave. Then again a yellow cab can get you anywhere.