score:5
Given that the Pronto fare system has the "Best value" option, i personally wouldn't care about buying a pass, but just loading money onto the card and using the public transport as i wish (as long as Coaster is not needed, which, since it's a commuter rail, most likely is not for the usual touristy stuff).
Explanation for the system (taken from https://www.sdmts.com/fares-passes/pronto-fare-system):
You may choose to purchase a Month Pass up front, or opt to earn-as-you-go.
If you choose the earn-as-you-go method, you will load money into your account, and then the appropriate one-way fare will be deducted on each trip. You will receive a Day Pass on your third tap each day. Once an adult rider has received 12 Day Passes (the equivalent of $72) during a calendar month, they have earned a Month Pass and the account will not be debited for the remainder of the calendar month.
In the PRONTO fare structure, a reduced fare rider will need fewer than eight (8) Day Passes to earn a Month Pass (equivalent of $23). A reduced fare rider would also earn a Month Pass on their 19th trip during the month (18 x $1.25 = $22.50 plus 50 cents charged on their 19th trip).
There is also an app for the system (called Pronto San Diego), which can replace the physical card, so you can even save the fees for the physical card this way.
The difference between Regional and Premium Regional only seems to be, that Premium Regional is valid on Rapid express routes, as well as the Coaster (only on Day Pass, taken from https://www.sdmts.com/what-fare-do-i-need).