If you are looking for a rabid attack or shrill defense of bevel up planes, both of which are common, stop reading. My thoughts are quite different, in fact so different that the reason I am writing this is to suggest that the debate seems to be meaningless.
In case you are wondering, I have and use both types of smoothers, though I tend to use a vintage bevel down plane the most for reasons that have nothing whatsoever to do with the arguments made against them, this one for example.
First, bevel up and low angle are often treated as synonyms, which is misleading. Bevel up planes are low angle in one sense: the bed angle is often 12 degrees. So what? The bevel angle on the blade and the angle presented to the piece being worked are both highly variable; Lee Valley sells blades from 25 to 50 degrees and ships the plane with a 38 degree blade. The wood sees the sum of the bed angle and the blade bevel angle, so this means it varies from 37 to 62 degrees. Therefore, any argument for or against bevel up planes on the basis of some fixed angle is meaningless, isn't it? I cannot see why the wood cares whether the bevel is up or down. 45 degrees is 45 degrees, isn't it?
What are the advantages of a Lee Valley bevel up smoother? It's simpler because the frog is fixed, and there is no chipbreaker. It is much easier to adjust the mouth with the front knob, versus moving the frog on a bevel down plane. It is quicker to remove and re-install the blade for sharpening. You can have a number of blades with different angles that you can interchange as the need arises. If you wanted to, you could have several blades with the same bevel angle so you wouldn't have to stop to sharpen during a work session. This gives the bevel up plane a lot of flexibility to go from end grain work to dealing with difficult grain with what is almost a scraper plane.
What are the advantages of a bevel down plane? I do like the light weight and balance of a vintage bevel down plane as well as the historical connection. The tote is more comfortable. I have nice ones that I paid $20 each for and refurbished. The thin blades sharpen easily and quickly, more than offsetting the time required to remove and replace the chipbreaker. I never feel the need to adjust the mouth so the inconvenience of moving the frog is irrelevant. The 45 degree angle presented to the workpiece works well for most applications.
So, having both, why do I use the bevel down plane the most? On balance, the advantages of them that I listed above outweigh the advantages of the bevel up smoother more than half the time. There is another dumb reason. I bought four blades for the bevel up smoother in A2 steel and have come to strongly prefer O1 steel for its ease of sharpening and superior edge. I haven't tried the new PM-V11 steel.
With the benefit of hindsight, I wouldn't buy a low angle smoother even though it is a great plane. The reason is that I have a limited tool budget, I think vintage bench planes are a tremendous value and I really like using vintage tools. There are so many great Lee Valley specialty planes that work extremely well and are superior to similar vintage ones, which are difficult to find in any case. Router planes, shoulder planes, plow planes, rabbet planes: this is where I find the higher cost of these new tools a good value. But that's just me.