I have the Shenhui SH-G350, which is commonly known as the blue eBay 50W (500x300mm) laser. I have been using it non-stop for the last 2.5 years cutting 1/8" & 1/4" acrylic, delrin, and etching aluminum and stainless steel (using a coating called Cermark). It has worked incredibly well. In my opinion, the key items to consider with these Chinese lasers are:
1. The controller. The Ruida controller (see link for the control panel below) seems to be the best supported controller, and works great with the 3rd party control software LightBurn.
https://www.ebay.com/i/122872943279?...r=423762208265
2. The control software. The Ruida controller comes with a program called RDWorks, which works pretty well. Even better, there is now a 3rd party software developed in the US called LightBurn (mentioned above), which is amazingly feature-rich and well-designed.
3. The water chiller. I bought an active (refrigerated) water chiller from a company in the US called LightObject, which carries many accessories for these Chinese lasers. This helps make the laser work more reliably and predictably.
https://www.lightobject.com/
4. What do you want to cut/etch? 100W will cut very thick material, but lasers typically make slightly angled cuts (due to the beam profile), so cuts on really thick material will not be as accurate as on a CNC router like the Xcarve. Also, higher wattage laser tubes typically won't produce as high quality images as lower power laser tubes, as you can't lower the power enough to get the full dynamic range for etching. Something to keep in mind. If you are primarily going to cut, then by all means get the largest tube you can afford. Acrylic cuts great on CO2 lasers, wood as well. However, most of the samples I have seen of etching glass (from other people, I haven't tried it myself) don't look that great to me.
It appears that most of these machines are very similar to each other, and probably made in just a handful of factories in China. Also, if the seller is in the US, they are almost certainly (unless they are a brand name like Rabbit or Boss) just a generic importer of Chinese goods. Look at their eBay stores and you will see a huge range of random products, from meat slicers to clothing. Lasers are just another product, so don't expect any support from them. As such, it is difficult to make a specific vendor recommendation. Just consider the 4 points above and ask yourself if you are handy enough to fix things that may go wrong (e.g. replacing a broken stepper controller, wiring, laser tube, power supply, etc.)