The main difference between a Smith & Wesson "Shield Plus" and an "M&P9 M2.0" is that the Shield Plus is a smaller, more compact version designed for deep concealment carry, while the M&P9 M2.0 is a larger, slightly more full-sized pistol with a higher magazine capacity, making it better suited for situations where concealability is less of a concern; the key distinction being the Shield Plus has a significantly higher magazine capacity compared to the standard Shield model due to its optimized design, while still maintaining a similar overall size.
Key points to consider:
Size:
The Shield Plus is noticeably smaller and thinner than the M&P9 M2.0, making it more discreet for concealed carry.
Magazine Capacity:
The Shield Plus has a higher magazine capacity than the standard Shield model due to its design, allowing for more rounds to be carried in a compact package.
Trigger:
The Shield Plus typically features a flat-faced trigger, which is preferred by many shooters for better trigger control compared to the curved trigger on the standard M&P9 M2.0.

size the shield plus is a smaller frame gun

The Shield Plus has larger capacity magazines.

Hi James and welcome. Essentially frame size and capacity with the Shield Plus having a shorter frame barrel. The M&P 9 M.2.0 comes in mid and full-size frames. So the Shield is a good compact Carry Gun and the M&P Shield more of a duty frame. Cheers, stay safe and enjoy! Either would work no issue. Just choose the best one for your needs.