I've owned all of the Ety line (except the bluetooth model) and could give you a quick comparison.
Both the ER6 and ER6i are very average in the budget IEM range. The ER6 is the better of the two, smoother and more natural than the 'i'. The 'i' has a lot less detail and a very modest bass hump, honestly it's pretty awful.
The ER4 range improve in every area. The spectrum is far more balanced, the highs controlled, the bass very deep. Instrument seperation is much better, imaging is nigh on perfect (best I've ever heard in any headphone). The difference between the P and S is a little complex though. The P is very very flat, every instrument represented like all the others. Etymotic's website talks about "improved bass response" in the P, I don't find that at all. The bass is simply inline with all the other frequencies. The S on the other hand sounds more natural. A/B them quickly and yes, you could say the S is brighter and has less bass. Listen to them for longer periods of time and you'll find the S to be more 'live' and the P to be dull and flat. This difference lends well to their preferred usage scenarios, the P great for 'on-the'go' use, easy to drive, 'in-the-backgound'. The S is for critical 'home/hotel' listening, good source (lossless) DAC etc, mind more on the music.