They almost all operate at the maximum value allowed these days. That wasn’t true back in 2013, but it is now. So you don’t have to worry about finding one which is better than another.
Remember that Z wave is intended to be a low power low traffic network. The way you expand coverage is by having more devices, as each of the repeating devices will pass along messages intended for others, sort of like the Wells Fargo pony express system.
The following FAQ should be helpful. Start with post 11 in that thread, read that, then go up to the top of the thread and read the whole thing. The topic title is a clickable link.
