You want to remove the Surround speakers instead of just adding a Sub to the system? 

The apartment size doesn't matter, just the room where the Sub will,be installed. How big is it, is it an open floorplan or enclosed room, ceiling height matters too,

It is my living room 400 sq feet. Itis a rectangular. 

For a very small room like that the mini Sub should do quite well.

Before you get rid of the surrounds maybe try switching them off and on and comparing how good surround media sounds without them active? I sure wouldn't give up my surrounds, f forced to choose I'd lean to having surrounds over having a sub, both is of course better.

I’m only watching TV. I have a 65 in Sony and most watch news, a bit of movies.  A bit of jazzy music. Which will you choose?  Tia

For news neither will really make a difference, surround audio will be minimal at best, same for the frequencies the sub responds to.

Movies, I'd want the surrounds first, then the sub but both if at all possible as they offer different enhancements to the Beam's sound that compliment each other.

Music, probably the sub first but I also really like the surround ambient mode for sitting and listening or full mode for parties or even just moving around the room or apartment, nice volume boost without pushing the drivers in the Beam.

so, the sub will be good, in your experience.  I’m in Canada and found a sub gen 3 for $ 600 CAD ( $434 USD)  Plan to sell my 2 x Sonos ONE and replace it with the SUB.

Just my two cents. Speaking from experience prior to this year's update to allow setting speech enhancement levels for ARC, Beam 2 and Arc Ultra; I would have recommended a Sub over surrounds. Prior to the speech enhancement update to set speech levels; Sonos soundbars IMO didn’t do a great job on their own to amplify the mid-range for speech. The sub would take over the low-end duties allowing the soundbar to better attenuate the mids and highs. 

Given your viewing/listening profile (shared) I’d say the sub is the right option. Although, I have to agree with ​@Stanley_4 that having both (sub and surrounds) is better. 

However, I recommend you consider the Sub-Mini given its price point versus a Sub Gen3 and your room size. That option may allow you to keep the Sonos Ones.

I have a full Sub in a 100 sq ft room, 10x10 and it is far more than is needed, have to dial the level back a good bit to keep it from booming me out of the room.

The full Sub will work in your room but you are going to have the volume even lower than I do for balanced sound.

You could get a Sonos refurbished mini Sub for around $350 US, that comes with a warranty and is far newer than the gen 3 full sub.

https://www.sonos.com/en-us/shop/sub-mini-b-stock

Totally agree with this vs. getting surrounds.  Granted this is a subjective thing, but to my ears, the QUALITY of the sound comes first … much more than the occasional ‘being able to hear the gunfire from behind you’ effect of surrounds.   I also strongly agree than in this small room the Sub-Mini is best.  (Frankly, I think the tighter sound of the Sub-Mini is better than the big sub regardless so I’d recommend that over the big sub almost every time anyway)

For that room, the Sub-Mini would have been a better bet, imo.  And like I’d said above, I think the ‘tighter’ sound of the Sub-Mini is better than the somewhat ‘sloppier’ sound of the big sub anyway.

Sadly the mini wasn't being made when I bought my Sub, trading now would be expensive for what seems to me to be a minor difference.

Which of these two models is the better choice for watching movies and listening to music, offering the best surround sound quality when paired with the Bear 2 soundbar + Apple TV 4K?

SONOS SUB Mini or SONOS ERA 100 x2?

Which do you recommend?

The sub-mini does not provide surround sound, only low frequency reinforcement. 

You have to decide which is more important to you. Or, both is a good choice.

Which do you think is better: Sonos One SL or Sonos 100?

The Era 100

After purchasing the Era 100, should I configure it in the SONOS app with the beam 2 soundbar as stereo or surround sound?

Depends on your intended use of the pair. My recommendation tends to be as surround speakers, but it still boils down to you, and how you intend to use them. 

​@Alec 33 

It appears you joined this thread as an interested party. I just want to make sure you fully understand the ramifications of your question.

I’m going to proceed on the assumption that you have (or will have) purchased two (2) Era 100’s speakers. You cannot configure one (1) Era 100 as a surround. 

When setting up two Era 100’s with the Beam 2 all three are Bonded as a unit:

• The default configuration is Surround Mode
• The Era 100’s cease to be independent and will not show as a stereo pair in the app.
• They cannot be used as Bluetooth speakers
• The line-in function to accomadte a CD player or Turntable is not available. 

However, within the app you can designate that when sending Music to the Beam 2 that the Era 100’s play in Ambient Mode or Full Stereo Mode along with the Beam 2. After the Music stops the Era 100’s will default back to Surround Mode.

To further explain:

• Ambient Mode is still stereo but at much reduced volume level. The Beam 2 will be the louder of the three speakers. 
• Full Stereo Mode is also stereo (as the name suggests) but at a higher volume level mimicking two stand-alone stereo speakers. In essence all three (3) speakers (Beam 2 and Era 100’s) play at the same level.

I hope this helps.