If you listen to both briefly, you will definitely find them quite similar. Although after longer periods, you will realise there are some areas of each headphone that really make them different. I will never mistake one for the other.
I'll start with the ESW10. It's a very good can, having a decent soundstage for a closed can (though not as large as the circumaural Denons I have tried - D5000 and D7000). Musicality and transparency is definitely there. A balanced sound from top to bottom, you never feel like it's thick at the midrange or top-heavy etc. I've never been good at telling PRaT, but the RS1 is definitely quicker, and more energetic. ESW10 is slightly too slow for some classical pieces, but classical isn't RS1's cup of tea either due to different reasons.
To say the RS1 sounds like an open version of the ESW10 is not completely wrong. There are a few things that contradict this though. With bowls, the soundstage is much airier than the ESW10. There is a very pleasing echo and "twang" that the RS1+bowls produces on acoustic/electric guitars that I have not heard any other headphone produce. However you will get the feeling that the music is a little upper-midrange/treble heavy compared to the ESW10. It tends to lack the body that comes in the lower midrange. Separation is better than ESW10 due to more air in the soundstage. Do note that soundstage is airier but not necessarily bigger.
Midrange is the RS1's biggest strength, singers sound like they're singing to you and you feel like your onstage with the band. This is also the biggest reason why some do not like the RS1. Too forward mids and the lack of a huge soundstage cause it to be quite genre specific, especially large scale classical pieces. For acoustic/vocal orientated music, I have not found a better headphone to my liking yet.
That extra treble energy and quicker bass (not necessarily lighter) gives the RS1 a very "electric" feel, making it excellent with both rock and fast jazz. It excels on slower music like acoustic/vocals as well.
The pads used on the RS1 is really up to your personal preference. For me, flats collapse the soundstage to almost 2D. The drivers are resting very close to your ear and that's what happens. The midrange, however, is even more intimate than with bowls and the texture is amazing. Treble energy is controlled, and you're less likely to get listening fatigue with flats. Bass is noticeably more than with bowls too. Normally RS1+flats go well with a neutral SS amp like Gilmore Lite, iQube etc whereas RS1+bowls go well with a warm SS/tube amp like Earmax Silver, Mapletree Ear+ HD, EC/SS etc.
Personally, I prefer the latter option because the lack of a decent soundstage is not something I'm willing to sacrifice. Try both out and see which you prefer. There's always the option of reversed bowls, HD414 pads etc.
RS1 definitely scales better than ESW10 from what I hear. The ESW10 is THE best portable can I have ever heard, and I prefer it to many circumaural cans (including the K701, DT880, DT990). However, it remains as good as it's purpose - for portable. The RS1 is a step above the ESW10 when properly amped. I believe that RS1 should be paired with a nice amp that controls it's treble energy and at the same time giving it more lower midrange for the body. Pair the RS1 with something warm and musical and you will get a intoxicating combination. Detail retrieval is definitely good on both phones, slightly better on the RS1 with some microdetails.
In conclusion,
Bass - RS1+flats>ESW10, RS1+bowls=ESW10
Detail - RS1>ESW10
PRaT - RS1>ESW10
What you do need to decide is how often you will be using each of them, for portable the ESW10 is the way to go if you have enough budget. For home enjoyment, no doubt get the RS1. They sound similar in certain ways, but definitely the RS1 is more forward and brighter.
Hope this helps.
