All else being equal and if I didn’t have bedbugs already … I would personally choose a protector over a full encasement because it is more breathable but of course this is also a tradeoff between the “risk” tolerance of each person and the differences between each type. If I actually had bedbugs then there is no doubt that I would use an encasement as part of the means to eliminate and protect against them (it will starve the ones that are in the mattress and protect more from entering because a bed bud infestation is usually not limited to the mattress).
It’s main benefit is that it reduces exposure to dust mites (and their “waste products”) and eliminates the ability of any bedbugs in the mattress to feed on you over the course of the night but the tradeoff is less ventilation and breathability because the material needs to have a micron size that is smaller than the dust mites and bedbugs or have no pores at all.
Some of the different types of mattress protectors (including the “membrane” type such as malouf) and the tradeoffs involved with each type are in post #89 here.
The main benefit would be that if you had a larger spill or “accident” that ran down the sides of the mattress that a five sided protector would protect the sides of the mattress as well (I spilled a glass of red wine once that went onto the side of the mattress and it required some effort and some “urgent action” to prevent the wine from staining the side of my mattress)
The tradeoff would be that it would reduce the airflow and breathability of the upper layers of the mattress to some degree which could result in some people sleeping warmer on the mattress.
Thanks for your input, Phoenix! So with the five-sided one, the main tradeoff would be more of the person sleeping warmer- but would it affect the mattress at all? Meaning, would this keep the mattress itself from “breathing” or airing out? Does a mattress need to?
If your mattress protector has a “semi breathable” membrane on the sides as well as the top then it would decrease the ability of any warm and humid air in the top layers of the mattress to disperse to the outside environment and airflow is a primary cooling mechanism in a mattress which is why it could have some effect on the sleeping temperature of the mattress for some people.