Luke’s Ultralite

No controversy. Did I mention they came from Ohio?

I was familiar with this jacket vs their silnylon rain jacket but I guess don't fully appreciate why silnylon the rain jacket couldn't serve both purposes. As you mentioned, Luke's describes the wind shell material as "This jacket uses a 1.1oz ripstop nylon that has been calendared for wind resistance and coated with a high quality DWR (Durable Water Resistant Finish)."

My assumption is that the nylon ripstop won't breathe and the wearer will need to rely on ventilation. If this is true, then there is obviously some property of the 1.1oz rip stop's wind resistance (per their description) that makes it appreciably more desirable than silnylon which I assumed is windproof.

Can you offer any opinions on this? Obviously my understanding of these materials is pretty basic.

"my understanding of these materials is pretty basic"
You and me both.

On a 0 to 10 scale with mesh being zero and plastic being 10, this material is maybe a 6 or 7 while sylnylon is closer to 9 or 10. Ventilation isn't enough to make up the difference. You'd sweat more.

I've had only good experiences with Luke's UL. Offers custom options for very reasonable prices.

I will note that he currently has a Pertex En shell for sale as a rain shell, while the fabric is not in fact waterproof. Either I misread the description or there is a typo somewhere.

I'm thinking that it is a typo. The description for the fabric reads the same on the on the Pertex site except "water proof" is replaced by "water resistant". The numbers listed are the same so it doesn't appear to be an attempt at deception. While it does raise the red flag,I felt that I was dealing with a very honest person. I expect a few quirks here and there when dealing with the independents.
At this time you are not going to find good rain protection that you will want to wear in dry weather. Everything is going to be a compromise one way or the other.

His Pertex shell does seem to be a step up on my scale from a wind shell and a step down from sylnylon. It appears to be somewhat of a niche product for when it's too damp for the wind shell but not damp enough to require a lot of protection. Ideally one would have the full range of shirts to cover every condition thus requiring only a single shell on a given trip. We could fine tune our gear every time. It also allows us to pick gear fine tuned to our particular areas.
Being lucky enough to afford one shell, I opted for the lightest he had. When the funds are replenished, I'll replace my Tyvek suit with a Epic rain shell. If I were in slightly wetter conditions, where I carried it more, I'd consider the Epic/Cuben fabric from Zpacks. I think I would still try to go to Luke for the build. The site says he does do custom. I don't know to what extent. Zpacks has a great reputation but we each have our own specialty. This seems to be Luke's.
That's just my thoughts though, and my made up scale. I would contact Luke and ask him. You'll get a fast answer. I have no doubt that he would steer you in the right direction. He has plenty of other fabrics to sell you. I doubt he actually makes much profit off of the material itself. He's selling his labor which is going to be pretty much the same regardless of the type of shell.

The question on the Pertex Rain Shell's waterproofness has came up a couple of times. I decided to send Luke an e-mail and will share his response.

In a quick response, Luke refereed me to the figures stated which are the same as the manufacturer's. With a rating of 1,000 mm it is considered waterproof by at least one article I found.
"For any fabric to be considered fully waterproof it must be able to withstand the pressure of a column of water 1000mm high without leaking. This is classed as a hydrostatic head rating of 1000…The British Ministry of Defense definition for a waterproof fabric is that it must resist a column of water at least eight hundred millimeters high."
A good article from Outdoorgear UK, with some fairly simple explanations.

http://www.outdoorgear.co.uk/hydrostatic-head.asp

I've determined, at least for my concerns, that the Pertex En was waterproof enough. While there are fabrics with greater resistance, other qualities are lost in the mix. They are heavier for one. This is lightweight backpacking.
After assurances from Luke that the fabric was indeed waterproof and was more than adequate for the water pressures exerted in the average rainstorm (300mm), I did a mild test by running water over the vest. The water did what it was supposed and ran off, leaving the fabric dry. Of course that doesn't account for hard wind driven rain or even the average rain storm, but this isn't a rain shell either.
It was enough to convince me to spring for the Pertex Rain Shell though. I'll do a shower test with that, unless of course I'm lucky and get rain.

I don't know if rain pants will be in the mix or not. Luke said he could do a pair of rain shorts or even a convertible pair, which would help with ventilation, but still durability becomes an issue. Perhaps a different fabric for the legs would help.

In the mean time, I did order some Frogg Togg rain shorts from Gander Mountain for $15 including shipping. They may be plenty good enough.