S3 has a very advanced engine for mapping notes to how they might be played on a guitar and many other features, but I haven't been able to make it handle more softly-played notes well at all. I've played [real] guitar for a very long time and know that varying dynamics are really key to bringing out the expressiveness of a piece. With S3 I feel like it is always playing them marcato, losing the relative emphasis that is so important.
A lot of players will play every note full-bore and ignore dynamics, especially with the gain cranked up, so this is just fine for emulating certain pieces, but I find it very frustrating when trying to get a more expressive dynamic range. You can get the Yngwie mid-shred easily enough with S3, but the Satch musicality is a lot harder to produce.
While H7 is a little klunky and buggy, and the tone isn't quite as nice as S3, the dynamic range and playability makes it more versatile as an instrument. They both have fine suitability for power chords although palm mutes are never quite what they should be. The strange truth is that for stuff outside the "it goes to 11" solos, I find H7 much more expressive dynamically. HTH.