In terms of protection (comfort is another matter, and everyone is going to be different) the only thing you need to know about either plugs or muffs is how much noise they attenuate, which is their NRR (noise reduction rating). NRR is measured in decibels (abbreviated as dB) - so a plug that has an NRR of, for example, 26, reduces the outside noise by 26 decibels. There's a difference in noise reduction depending on the actual frequency of the sound in question, but the standard procedure for determining NRR takes that into account.
Simply, the higher the NRR the better. Foam plugs alone actually do a very good job and many have an NRR of 30 or higher. Many muffs alone do not do as well. Doubling up of course is the best, but (not that it matters a great deal) the NRR is not additive - in other words, adding NRR 24 muffs to NRR 30 plugs doesn't result in an NRR of 54. Remember that the decibel scale is logarithmic, so every 3 decibels represents a doubling or halving of sound energy, and the difference between an NRR of 27 and 30 may not sound like much but is significant.
The particular Howard Leight muffs shown in your post have an NRR of 22 - that's not particularly good. In general, all of the less expensive electronic muffs have comparatively low NRRs, and even the high-end electronic muffs rarely equal the protection of either plugs or good quality non-electronic muffs. That said, if you always plan to wear them doubled up with plugs they'd probably be fine. That assumes that you wear both the plugs and the muffs properly, of course. I know some shooters who double up and turn the volume all the way up on the electronic muffs so they can hear range commands and also for general range awareness. That's a good strategy, but you can sure go through some batteries that way - I use a pair of Peltor Tac 7s, and they just eat up 9V batteries.
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