First of all, I find the quality of the picture on the WDTV devices and Google TV hardware better than when using the ipad streamed through the AppleTV plus sling still needs to fix these ongoing SD/HD streaming issues with the iPad. It's getting annoying, especially on the HD setting. Of all three that I use with the slingplayer app, the WDTV Live, the Sony GoogleTV box, and the WDTV play, I'd say the Sony is the easiest to control the slingbox. The WDTV live second and the WDTV play third. Reason the Play is third is because of the size of the remote and there's no number keys on the remote to change channels easy as it is with the WDTV Live. ( tried switching remotes using the live remote with the play box. Too many things don't work like channel changing using the numbers on the remote ) You have to change channels with the onscreen menu on the Play. The Sony's remote has the advantage of using the touchpad to click the menu buttons on the screen, rather than arrow up down left and right which can be tedious on the WDTV boxes, but none of the connected devices that run with the 500 have a great user experience when it comes to DVR stuff where you have to fast forward and access DVR settings to delete programming, etc.. The delay is pretty bad on all the connected devices, so fast forwarding is a hit and a miss due to the time lag, and the farther away from the slingbox, the longer the delay and the greater chance of <FF> going a minute past the recorded program after pressing the play button. You do get used to it by guessing the time delay and if you happen to be early, you just let it run. Also, there is a buffering issue bug that happens when you keep using the remote functions; the buffer gets full on the connected device and the delay gets longer and longer. Hoping for a future fix on that one.
So, would I recommend all of them for LiveTV. Yes, because there's so little that needs to be done to get your slingbox into action. Run the slingplayer app, turn on the cable / sat box from the onscreen menu, change the channel and you're good to go. The sling stream quality is pretty good for being 5000 miles away from my slingbox and it's been pretty stable on all three of my slingplayer connected devices ( 4 if you include the slingcatcher ). You're not going to get a slingcatcher experience from them ( It's still the Rolls Royce of connected devices, but unfortunately it doesn't work with the 500 ), but if you keep your expectations in check, you'll still find them all functional. Would I recommend any of them if you use it with your DVR to fast forward through the commercials of recorded content. That one all comes down to how much patience you have.
If you're leaning towards the play, I can say that it's a watered down version of the live, but the U.I is faster and it's easier access to get to the app your looking for since you can create your own personal my favorites page on startup. Outside of a few codecs that don't run ( DTS, MPEG ), it plays everything that I have in my collections ( avi, mkv, mp4, xvid, etc ), so my old Roku box is now gathering dust as I now use the live and play for netflix use.
The slingplayer app on the Play will do the job, but you'll have a few tearing your hair out moments as you try to factor in the buffer problem and the delay aspect of a remote slingbox. Also, if you're running your slingbox remotely, I'd recommend you make sure your slingbox has been setup for a TCP stream rather than 'relay' which is what you'll get from your connected device if the port forwarding hasn't been setup correctly on the router. The reason for that is, from my experience, TCP is more stable than Relay on a connected device. You might get a little bit faster speed on relay, but the slingbox will lose a connection easier or crash easier than it does with TCP through a connected device.
My advice is buy the Play from a place like Amazon with a good return policy and experience the slingplayer app on the WDTV box to see if you can tolerate its shortcomings. If you can, then it's a great, inexpensive little box that offers you more than just access to your slingbox app.