Shields/Shield Plus get a bit tricky with this for a couple of reasons:

Originally, they all had a standard RMSc footprint with the back posts. There was a rolling change to the Holosun K footprint, which is a modified version of the Shield RMSc footprint with slightly shorter front posts and no back posts.

There are spacer plates to let you use a Holosun K footprint optic on a standard RMSc footprint.

However, an added issue is that some Shields and Shield Plus have a negative forward cant that can make zeroing a Holosun optic impossible. My Shield Plus had this issue when I tried to mount an EPS Carry, which is regarded by many to be the best red dot in this form factor. I ended up sending it to Nameless Armament to get the cut corrected to the Holosun K pattern and leveled.

My choices in RMSc/Holosun K pattern optics:

Open emitter: Holosun 407/507K, Vortex Defender CCW Micro, SIG Romeo-X Compact
Enclosed Emitter: Holosun EPS Carry, SIG Romeo-X Enclosed Compact

If you go for the Vortex, make sure to get the updated model with the 10 minute auto shutoff, not the 14 hour auto shutoff. The updated model reportedly has a better LED emitter that's more efficient in spite of a faster refresh rate.

If you want assembled in the USA, SIG is the way in this size format. The flip side of that is that they're the most expensive and the South Korean sourced LED emitters don't have as long of battery life as the Chinese emitters.

With the Holosuns, the 507K will give you better battery life (especially with the circle dot reticle) simply because they use a bigger CR1632 vs the CR1620 in the EPS line. The EPS does, however, have better optical clarity in addition to being more weather resistant by virtue of having an enclosed emitter.