It really hard to say which is better. You see test on testing sites where what is inside the units is exactly the same but they get different results. There are only a very small number of manufactures of the actual chips.
What is really bad is one testing site will get one result and another will get a different. This clearly points to the main issue with any testing review of device like this or wireless. Your house is going to be the determining factor. Since nobody tested in your house there is no way to know how your house wiring will work with any particular unit.
Hard to say how you select these devices. The speed ratings mean they meet the same standard and are suppose to actually be able to be inter operable between brands. Best advice I guess is to buy from a company that has a good return policy so if you have major issues you can return them.
Most times you will not see random delays or lag on powerline that is more of a wireless things. You normally will get the fastest speed they can run and it will always run at that speed, it generally when the devices are activated quickly tests to see how fast it can run with no data loss and select that as the speed it will run.