Without knowing more specific + or - details about the possible choices, I’d pick Shapiro over Kelly when you add up all the variables:
As a governor Shapiro has executive experience, which Kelly doesn’t. Cooper is also a governor but is already 67.
Pennsylvania is an absolute must-win state. Arizona is also very important but is not a must-win. Gotta get PA.
Shapiro is only 51 so would still be plenty young enough in 8 years to run for POTUS.
With Shapiro you’re not taking a current Senator off the field like with Kelly. Yes the Dem gov will appoint another Dem but will that person (or other Dem) be able to win again in 2 years vs. keeping Kelly in place who is highly likely to be re-elected. We really need to build up a bigger majority in the Senate over the next couple of cycles to be able to get important things done.