If you don’t care about Dolby Atmos audio or higher resolution audio formats like Dolby TrueHD and multichannel PCM, then consider adding the Sonos Sub and/or Play:1s or One SLs to the Beam rather than upgrading to the Arc. Most people would recommend adding the Sub before the surround speakers. Every good home theater system needs a good subwoofer to produce the low frequency sounds that give you a rich, more balanced, and powerful audio experience.
But if you watch a lot of Blu-ray discs or want to enjoy Dolby Atmos content including Dolby Atmos music, consider upgrading to the Arc. Lossless audio from Blu-ray discs sound pretty incredible from the Arc.
The new Beam (Gen 2) is another option to consider although it won’t be as dramatic of an upgrade as the Arc.
Others factors to consider is room size, ceiling height, and distance you are seated from the soundbar. The Beam is more suitable for smaller spaces. The Arc is more suitable for larger rooms. If you have really high ceilings, vaulted ceilings, or if your main seating position is 8 or more feet away from the soundbar, you will not really hear the height effects produced by the two upward firing drivers when listening to Dolby Atmos content.