I have got x16 pro irons and have found them to be the best irons I have ever owned. I think I have won more with these clubs than any others I have had. That said, it has always felt like the shafts were the best bit about them as they really seem to suit me. They say "Tour Stiff, Constant Weight, Callaway Rifle". Not sure what this really means in English but they have been perfect for me.
I have seen the non-pro versions of Callaway irons and don't think I would like to play with them as the heads are massive and have considerable offset - both aspects that I don't like at all.
I tested X16/18/20s about (um....) a year and half or so ago and really enjoyed the process.
The X16 are probably one of the best models ever made, but o.m.g. they are fat and ugly. I know people think all the X series are total shovels, but the 16s are the most shovel-est ever made, excluding Big Berthas.
If I was going for 16s, I'd defo go with the pro model.
The X14 were the last Cally set I thought didn't look large and clunky. That said the tour or pro version of subsequent releases have looked good although they won't be as playable as the bigger standard model. I guess it is like any set though its about trying them and seeing what works for you