Arcteryx Beta AR vs LT

CAUTION: Beta LT just got revised in Feb 2021, and this review isn't updated yet. 

Let me give you a short answer first: Arc'teryx's Beta line products (SL Hybrid, LT, AR and SV) are focused on versatility and all-round use. You can see this in Arc'teryx's Naming Scheme Page here as well - for both "LT" and "AR", as well as "Beta". 

• AR (All Round) as the most well balanced and hence the center of these 4 jackets,
• And the LT (Lightweight) as the lighter model of AR {345 grams (12.2 oz) vs 455 grams (16 oz)} - with %10-15 breathability, %10-15 weather resistance and %25 durability sacrifices when compared to the AR. 

So what's the take-away here? 

If you're an ultralight traveler/backpacker, then the lighter weight and more packable structure of Beta LT can most likely be a more suitable option for you. For all the other folks (probably 90% of you?), Beta AR is probably the way to go. 

You can see their side-by-side comparison on Arc'teryx's site here. 

Now the longer answer: 

The biggest difference between these jackets is in their different Gore-Tex fabrics.

N40p-X (Beta LT) vs N40r-X & N80p-X (Beta AR)

Let me clear this first: N40r-X & N80p-X of Beta AR aren't used simultaneously throughout the entire face of the jackets.

Instead, the N80p-X is used in high wear areas that are better reinforced: Shoulders, back, forearms and top of the hood.

In order to prevent the weight and price of the jacket going through the roof (N80p-X is very heavy, robust and expensive), N40r-X is used throughout other areas where less durability is more acceptable. 

This is also why Beta AR is the most well-balanced of all Beta jackets because Beta LT is 40D and Beta SV is 80D everywhere (they aren't balanced but rather intended for more niche uses).

Beta LT is equipped with N40p-X everywhere.

So what do the numbers 40 and 80 exactly mean?

If you're unaware, the number after the first letter represents the denier (weight) of the fabric. It's usually referred to as 40D and 80D (denier), etc.

As the denier increases (which is 40 vs 80, in this case) the durability and weather resistance will increase, but the jacket will also end up being heavier, more stiff (and hence less packable) with a less pleasant touch.

You can now see the main difference between Beta LT and AR - but let's continue, more to come.

The letter (p or r) after this number represents the fabric weave. R stands for ripstop, which, unlike plain weave (P), is woven with a grid texture.

This is the only difference between them. And as a result:

• Ripstop weave (Beta AR) is expected to prevent the potential tears from propagating considerably less then the plain weave by blocking their paths with elevated fabrics. 
• Whereas the plain weave (Beta LT) is expected to bead the water droplets better than the ripstop because unlike the ripstop, the pathway of droplets won't be crossed by elevated fabrics.

This outcome kind of makes sense when you think about it in theory, but is minimal (or almost non-existent) in practical gear use.

Because:

• Gore-Tex 3L Pro is already very high quality by itself, so ripstop doesn't really add much to Beta AR's durability in practice.
• Both jackets are already DWR coated, so the plain weave of Beta LT doesn't really cause a noticeable difference in water beading.

So, as you can guess now; the vast, vast majority of the extra %10-15 weather resistance & %25 durability of Beta AR and more packable & lighter structure of Beta LT result from the denier difference (40D vs 80D) - NOT the weave difference (ripstop vs plain).

But what about the %10-15 breathability difference?

Breathability

This one will be much shorter.

LT is much less than the AR because it doesn't offer mechanical venting: LT doesn't come with pit zips, whereas the AR does.

If, on the other hand, you want something much lighter and more packable than the AR, and you can sacrifice some breathability (LT doesn't come with pit zips unlike the AR), durability and weather resistance in return, then the Beta LT would probably be your best bet.