I haven't read much good about the Chinese lasers. They seem to be hard to set up and difficult to trouble shoot. Having said that, I have also read a FEW reviews that were very happy with the laser once it was finally set up and all the troubleshoot was done.
Power: the more power the the thicker & harder the material you can cut. However, A GREAT many people do extremely well with 30 & 45 watt lasers. All lasers engrave awesomely unless there is something wrong. Alot depends on what you are planning to do, and how much space you have. Exhaust removal is a very important item consideration.