Hi deception,
You have narrowed things down to a choice between “good and good” which means the choice between them would be a matter of your own personal value equation because your choices are also quite different. I can’t make an specific comment about their relative value without knowing the size you are looking at and whether the Mezzanine is a mattress only price or the price for a set including a foundation.
The Alexis is 6" of latex and the Mezzanine has 4" of latex (the value of the Alexis would be more) and both have a high quality base layer (a 6" high quality HD polyfoam core for the Alexis and an 8" bonded latex/polyfoam core for the Mezanine).
The Mezzanine would have a very different feel though because the latex topper is 14 ILD blended Talalay which is the softest they make and it also has an inch of slow recovery latex under this which would have a different “feel” than fast response latex.
The Alexi uses a minimum of 19 ILD latex (most manufacturers choose not to go as low as 14 because of potential durability issues because even though it’s more durable than an equivalent polyfoam … it’s not as durable as firmer latex). Its thicker layer would make it softer than if it had only 4" and it also has soft quilting foam on top but even so I doubt it will feel quite as “plush” as the Mezzanine.
The Alexis also has the option of exchanging a layer if it turns out that you want to change your comfort choice after sleeping on it for a while.
Of course there is also the extra “value” of a local purchase that you have tested in person and some extra “risk” for any online purchase that you haven’t tried that also has to be factored into the mix even though the comfort exchange lowers this risk.
Like a replaceable layer … the topper also has the advantage that it can be replaced (just like a loose layer in a zip mattress) without having to replace the entire mattress. A latex topper typically wouldn’t have an issue with shifting.
So I can’t tell you which one to choose because that’s up to your own preferences about the differences between them and only you can know which of the “tradeoffs” are most important to you but at least you have a sense of the differences between them. With the “missing information” about size and whether it’s mattress only or a set it would also make a comparison a little more “apples to apples”.
It’s always a good thing when you have eliminated all your worst options and you are only left with good ones 
Phoenix