These are fantastic but I'm not retiring my 120's.

First let me say that I suffer a serious hearing loss which makes it difficult to watch television with my wife because I have to have the volume up so loud and even then I can't hear everything. I have had two sets of sennheiser RS 120's for a few years now and loved them. My wife wanted to get me these for Christmas this year because they were apparently capable of surround sound. Now I'm not an audiophile but I will try to point out some pro's and con's here.

The 175's are an over the ear fit as opposed to the 120's being an on the ear fit. The 175's fit much tighter which is very nice when up and moving about yet the tight fit makes it difficult to hear when my wife is speaking to me during a show which she does quite often. I'll let you decide if that's is a pro or a con. The 120's were loose enough that I could hear her speak even though I still had to remove the headset to understand her. The looseness had a downside though in that if I went to the fridge for a drink or snack and bent down the headset fell off. Not so with the 175's. Both sets have excellent sound reproduction even though the 120's are analog. The 175's can be digital or analog which can be decided by a switch on the back of the transmitter. You have the option of connecting to the sound source by optical cable or 3.5 jack which in my case required a y adapter converting to RCA jacks. I was amazed to find that there is a very noticeable difference between digital and analog, digital being the best by far. With the digital you don't hear any static or interference noises in non-dialog or silent spots. At least I couldn't hear any and my wife said in her words "this is fantastic".

sound with the 175's:
You can switch between regular stereo, low SRS, and High SRS by clicking a button on the side of the headset. This can also be controlled at the transmitter. There is a very noticeable difference between each setting. I have noticed that on low SRS the background recordings seem to be moved back and reduced in volume while the main dialog remains at regular volume. Ever had one of those instances where the main character is talking in a whisper voice, which seems to be in vogue these days, and is suddenly overrun by the background noise, which also seems to be a problem these days. Well, low SRS helps to overcome that problem for me. I can hear the dialog very clearly and if you've lived for a while having to constantly asked "what did he/she say?" then you can really appreciate that fact.

In High SRS there is a distinct surround sound quality. I've never been able to experience this before due to my hearing loss so to me it is awesome. I love Paul Simons music, especially "Graceland", "father and daughter" and "late in the evening". I just thought I liked them before but after listening to them through the 175's I have heard notes and instruments that I didn't realize were there. Now I love the songs. Having discovered that I went back and watched a couple of my favorite movies and discovered background sounds that I had been missing and they add so much more to the entertainment value. Keep in mind that I have experienced all this with the 120's also and there is a large difference.

continued pros sound aside:
I like that the controls are on the side of the 175 headset because I often recline when watching television and this keeps the controls from contacting anything and changing the settings unintentionally.
This was a problem with the 120 sets because the controls were behind the headset.
The 175 set, to me, is much more comfortable because of the padding and because possibly of the tighter fit which keeps them in place verses the loose fit of the 120 set.

cons (maybe):
The 175 set is only capable of employing two headsets.

The 120 can employ several at one time. I'm not sure how many but certainly more than four because I have that many headsets.

The 120 has an option of three channels selectable on the transmitter and the headset which means you can use more than one in a household without interfering with each other. This is the situation at my house. I have one set in the bedroom and one in the living room and we can use them separately on separate channels at the same time. My grandkids love the headsets so this settles a problem when they come to visit and want cartoons while I want news.

The 175 has only one channel so this setup would probably not be possible.

The 175 power button has to be held down for about three seconds to activate while the 120 has a slide switch. The 175 has a very slight delay between activation and transmitter connection. The 120 is instant. This caused a rather humorous incident early on. My wife was watching a movie on Netflix and I picked up the headset, turned it on, placed it on my head and thought I was waiting on a connection when in reality I was in a very quite point in the movie just before an explosion and massive gunfire. I was on my way to my recliner when all hell broke loose and it was so realistic I almost wet my pants and overran the recliner. It was apparently the highlight of my wife's day. She still laughs about it. Her lack of concern about my bruised shins and ego aside...after the fact it was funny.

I know this was lengthy but I hope it helps someone in my situation to make an informed decision.

In short I would never had purchased the 175's for myself due to the cost because I wouldn't have believed they would make this much difference. Now that I've experienced them I would buy them in a heartbeat.

Now a word about sennheiser's customer service. I had a situation early on where I lost the optical connection with my TV. I was quick to blame the new headset simply because they were new. I sent an email to sennheiser in hopes they could find a solution before I had to send the set back. I had a return mail in about 45 minutes telling me what to do. Turns out the problem was with the television and not the sennheiser transmitter. Quick response, great customer service.