I can't answer that as I don't know what WMP and JRMC are but I'll tell you what I like about the Squeezebox and you can decide how they compare.
*Beautiful aqua fluorescent display. I can't understand people who try to control audio/video gear with a display built in to their remote control... the Squeezebox display shows album/track info, EQ and other "visualizer" info, displays home automation messages from my stargate, displays date/time or RSS feeds when it is off, etc, and everybody in the room can see them! It is also much more satisfying to have a display in each room that shows specific song info or Internet radio info than it is to have a keypad that just selects a source and volume level.
*Uses a Burr-Brown DAC for true audiophile performance. Some of its competitors use lesser electronics with noticably inferior results (they sound like you were playing your music through a computer sound card!)
*All players can play in sync or they can all play independent programs (or any combination). Nice for parties. Also nice for everyone to go off and do their own thing.
*The server software is free. It is actively developed and very nice. Even ROKU, one of Slim Devices competitors, requires that you use Slim Server software in order to enable advanced features like synchronizing players.
*There is a free software emulation of the Squeezbox players. You could use Slimserver and Softsqueeze (the emulator) and never have to buy anything. (Except you would be playing your music though a PC sound card which would suck for your main system).
These are the main reasons I like the Squeezebox but there are a lot of other good features that might interest you. Check out the
Slim Devices site for a lot more info.