Ah, so many people ask for "decent sound" not realizing that it is totally subjective. For some people, a sound bar is fine. For hi-fi audio enthusiasts, it's not a consideration. A sound bar won't compete with an AVR (audio video receiver) and separate speakers and sub-woofer when it comes to sound quality, so it comes down to your goals and expectations. If that is a wall unit to hold the TV, you may not have space for an AVR and floor speakers. What you gain with the Sonos is a lot of convenience. You can stream to the Arc with the Sonos app or Apple Air Play for music so you just need a cable box or TV arial. The sub and satellites are wireless which eliminates speaker cable. For the 2 main seating positions in front of the TV, the atmos surround effects can be pretty good.
For that size space, the little Sonos sub is not going to give you the home theatre experience. It will provide the bass that the sound bar is lacking, but don't expect much deep bass outside of the living area (depending on placement). And with the open concept, varying ceiling heights and windows and glass doors, the room acoustics will be all over the place any way. For every day TV and background music you may find the Arc system is ok. Web site shows retail $1,856 for the Surround Set; less for just the bar and sub.
Next step up is an AVR with book shelf speakers, centre speaker and maybe satellites and sub-woofer. You can get a good AVR for $500 (Denon and Yamaha are popular) leaving almost $1,500 for speakers, if we use the Arc system as a starting point for budget. Others can recommend some decent speakers if that's something you would be interested in. That would improve music quality a lot, but involves careful planning of speaker placement and wiring. You can do the whole thing in one shot, or prewire where you want the speakers to go and get the mains and center to start and add the satellites and sub-woofer later if you want to build a better quality system in stages. Music streaming would require a smart TV, separate steaming device or a Roku or similar device plugged into the TV (might need a TV with one HDMI-ARC connection and an AVR with HDMI-ARC support). Other options there too but Sonos has a pretty good reputation for supporting various streaming services.