The problem is not the synth, It's the way how you play it. A synth / sampler can capture all the sounds you can ever imagine but if you play it through synth style of playing you will loose a lot of characteristics specially guitar. Try to compare playing a good guitar sample from your favorite synth and from midi guitar using the same sample you will notice the difference. Same in sax sound using midi wind controller versus playing from synth keyboard. I'm thinking of an idea of putting a six-string controller on the keyboard panel and see what its effect.