You are correct that the 3.8W laser has a slightly larger spot size. When considering your choice, you need to really think about your applications you will be doing. For some, the larger power and larger spot size is beneficial to people who are doing engravings where you don’t need to extra small resolution. Let’s take an example of a large logo. With the 2.8W laser it will have a smaller spot size and will need to go back and forth across the logo more times to cover the full image of the logo. It will also have a about a 2x less power than the 3.8W at recommended settings. So, combine the two together and you will go about 3.7 times faster engraving a large logo with the 3.8W than with the 2.8W laser.
Cutting is really just about the more power. At the recommended setting of 2.5 amps you will get above 3.8W (the lasers we are shipping now are coming in at about 4.2W). The 2.8W at 1.5amps is probably going to be around 2.1W or so. So, you are getting about 2X the power for cutting which means it will either cut at 2X the speed or it will cut 2X the depth at the same speed.
If you want to do very small engravings or super high resolution pictures, then the 2.8W laser is for you. It will do a 4 point Arial font. For pictures, you will be able to just barley see the pixels when holding the picture in front of your face. The 3.8W you will be able to tell. You have to hold it farther for it to become less pixelated. The 3.8W can do a decent 8 point Arial font when focused properly.
Anyway, hope this helps! Let me know if you have specific questions. You can reach me at firstname.lastname@example.org