Phil, take a moment and figure out what your primary use for the laser is going to be. Depending on what you want to do the 2.8 W unit might be a better fit to your job than the 3.8 W unit.
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Bigger is always better, right?
Using the numbers on the Jtech site:
2.8 W has a spot size of 0.009 inch 2.8 / PI * (0.009 / 2) ^ 2 = 44013 watts per square inch
3.8W has a spot size of 0.015 inch 3.8 / PI * (0.015 / 2) ^ 2 = 21503 watts per square inch
You actually get higher power density (hotter spot) at the material with the smaller laser (with a smaller spot - good for engraving detail).
Others with experience with the laser diodes can chip in here, but you will not be happy trying to cut materials with any laser diode.
The Opt people have a good web page (keep in mind these test were done with a 6 Watt laser your results would be roughly twice (3.8 w laser) as many passes to do each cut). They used to give times, but I see that they have changed that (really pointed out how slow cutting is).
Very thin materials.
When you say faster, do you mean the same job will complete in less time with the 3.8 rather than the 2.8?
And, is that just because the 3.8 burns a wider path than the 2.8?
Tried cutting a 3mm birch with my 3.8w JTech before, it took at lot of passes and sides are pretty charred. Jay said air assist might help but not much. I have not tried it with air assist though
The 2.8 watt unit delivers higher optical power to a thinner path than the 3.8 watt delivers a lower optical power to a wider path.
Think this way 2.8 watt is smaller diameter mill with greater depth of cut capability and the 3.8 watt is like a larger diameater mill with less depth of cut capabilityā¦
One thing I did was to get a new lens for the 3.8w laser that gives me a much smaller focal point. Iāll find the link of the lens I used.
Just as a reference Phil, I have been quite surprised in reading the forums of people that use the K40 Chinese 40 watt CO2 laser cutter. Although, they do a much better job of cutting than a laser diode, Iām still not impressed by their cutting capability.
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Youāre going to get hooked on doing laser images.
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@ErikJenkins What lens was that? I ordered this lens for the 3.8w but have been unable to get it to focus. It seems to make a 1" long rectangle. I can get a smaller focal point but its at the very end of the threads and falls out with any movement.
It sounds like the lens is in backwards.
Edit: I purchased a 3 element lens. I did the same thing when I swapped out the lensā¦reversed it and focus was easy 
I installed it with the threaded end in first. I have not tired it with the notched end in first.
mine had a threaded housing (brass end) I unscrewed it from the brass ringā¦flipped itā¦it is a apparently common error.
This is how mine cameā¦I took off the ring and put it on the other sideā¦
Also curious about the lens. I have the 3.8 and have not used it much because it takes too long to do anything other than line artā¦ Maybe I still donāt have my settings right or havenāt focused it properly but it took a good 8-10 passes at 40ipm to cut through some thicker cardboard and text outlines are the only thing I can accomplish unless I want it running all day. Seems if I run any faster than 40ipm it doesnāt come out well.
So flipping it may work better?
No, if you are cutting at all you have the lens in correctly. These laser just do not cut well at all. They are primarily for etching and they do a great job at that.
If you are having a problem getting a good focus, I found using a piece of black anodized aluminum makes it much easier to see the focal point. I use a piece 0.5mm thick, focus on that piece while it is resting on the material I am etching. Then drop the z axis 0.5mm and I am good to go.
I got this guy, itās 40watts and not enough power for me 
Donāt forget to wear laser safe glasses with those open models and make sure you learn what NOT to cut or engrave with a laser, like vinyl (it can easily kill you).
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Be sure to set things up so that your kids (or anyone else) canāt walk in while youāre working the laser.
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JTech does have some inspection window filters, but they are small. You have to worry about reflections as well.
When I put my laser setup together it will be totally enclosed with an inspection window filter. Iām pretty paranoid about this. Iāve worked with lasers for a lot of years in other applications. Itās so easy to make a mistake that cannot be undone.
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Thatās what your Last name stands for, got it. 