Presumably you're using the 3Nm stepper motors which draw 4.2A? I've found them to be OK on one 500VA transformer, but more is more stable.

The PS806 is 500W (500VA strictly) and a linear PSU which for this application is generally considered better than a switching PSU which is what the SPS705 is. In addition the SPS705 is only rated for 340W.

I wouldn't consider either. All you need to find is a suitable transformer, then buy a bridge rectifier, capacitor(s) and solder it all together which is easy enough. If you got the 625VA transformer here that would be good for 4 motors. Shop around, that may not be the cheapest. Anything with that VA rating or above is fine, 24V with two secondary windings in series will get you 48V which is fine, similarly 25V/48V/50V transformers are fine. If you decide to do this I can post the circuit etc.. or you can probably find it as I've posted about this many times before.