Here is a detailed comparison of the two systems:

https://www.rtings.com/soundbar/tools/compare/sonos-arc-with-sub-one-sl-speakers-vs-sony-ht-a7000-with-speakers-bass-module/15256/28281?usage=8767&threshold=0.10
 

Yes, Play:1s and a Sub (Gen 1) will work with the Arc.

Yes, and you can add a 2nd sub if you add a Gen 3 sub.  As to how it compares to Sony, I have no experience with that. 

I think I need to go and listen to both systems.

The recent issues with the ARC dropping surrounds and the Move problem's on S2 have got me a bit weary really.

S1 has been very reliable up to now.

Hi 

I don’t have any experience with the Sony HT7000. Judging from the comparison link submitted by @GuitarSuperstar the only reason I can see to go with the Sony HT7000 and surrounds is the ability to output DTS codecs directly and BlueTooth compatibility. 

The Arc relies upon the TV to be able to pass-through the DTS codec to it. According to the comparison the Sony HT7000 may play a bit louder; but IMO that is subjective to the listener.

All other things between the two being equal or tipping in favor of the other (slightly)...you might consider the following….

Looking at other benefits of the Arc, Sub and One SL x 2 as surrounds:

1. Easy intuitive setup via the Sonos S2 App for iOS and Android
2. Multiple music streaming services to choose from integrated via API with the Sonos S2 App 
3. Choice of Alexa or Google Voice assistant
4. Ability to sync with other Sonos speakers that may be installed in other rooms for whole home music integration
5. One SL’s can be used as surrounds or stand-a-lone speakers elsewhere in the home for increased versatility
6. Sub if not used with the Arc can be bonded to other Sonos speakers dedicated to music either as a single speaker or stereo pair (i.e. Sonos One, Sonos One SL, and Sonos Five)
7. Two subs can be bonded to the Arc to eliminate dead spots for LFE to compensate for irregular room shape or just to add more thump (one sub must be a Gen 3 and the other can be a Gen 1 or 2)
8. Sonos speakers are typically designed for a 4- 5 year (maybe 7 year) cycle before a replacement model is introduced. However, Sonos still supports the product for many years beyond EOL until software/firmware limitations make them obsolete. The Play:1 introduced in October 2013 is still supported even though it was replaced by the Sonos One in October 2017. The Play:1 can still be used with the Arc as a surround.
9. Sonos products maintain their value and can be sold on bidding sites for as much as 60-70 percent of their original retail value upwards to 3-4 years after introduction (I speak from personal experience).
10. Sonos typically offers 15-30 percent to upgrade speakers within the same design/use class when a new model is introduced.

I don’t work for; nor do I own stock in Sonos, so mine is strictly my own opinion. Sonos is not perfect but it checks many of the boxes for me. Considering how many Sonos products I own I probably should buy some Sonos stock 

If I went for an ARC I think the HD Fury would have to be used for now.

They do a 4 port HDMI switch with eARC output from what I've seen. 

Agreed. In order to send DTS to the Arc direct from the source. However. the Arc even using the HDFury will still only process basic DTS. So something to consider as well.  

I suspect to get a true objective view on a non-Sonos device, you should be looking at boards that are not sponsored by one of the companies. I take it as a give that people here will have an inclination to like the things that they already have, since they’re part of this community. I’d be going to avforum.com, or some other place (rtings for a non-community resource) for more unbiased opinions…..and a greater chance of people having tried both devices. 

I'm still looking at both sides, obviously the ARC would  save me buying Sub and Surrounds again.

I suppose it hinges on DTS-X and if that will ever be a must have format, it does seem most 4k Blu-ray disks have Dolby Atmos. 

As for music I mostly use Spotify so that is supported on both.

Thanks for the replies, all food for thought.

Well still looking, Samsung HQ-950A has cropped up as well (11.1.4). Front side and upwards firing surround speakers seem a really good idea.

If Samsung wasn't covered in cloth it would be a good option.

Think I'm going to hang fire for now and see what happens with next year's new additions.

No point rushing, those Samsung rear surround speakers look an excellent idea, maybe a metal grille for next year..

I would just personally choose to go with the Sonos Arc, particularly if I had previously purchased two potential Sonos surrounds and a Sub already, but also because of the availability of being able to return the product, as detailed in this link below, that’s if not entirely happy with the HT product:

Sonos 100 day return policy / Your right to withdraw the purchase

Anyhow, I do have the Arc, surrounds and Sub already and there’s certainly no complaints here from me, or the family. It works/sounds great with our LG C9 TV. (No regrets at all).

Is the ARC significantly better than the Playbar?.

Still on the fence!.

Yes.  And if you have a flat 7-9 foot ceiling and sit at an optimum distance* from the Arc, Dolby Atmos is magnitudes better than 5.1.   I watched the season finale of Stranger Things last night in Atmos on an Arc, Sub and One surrounds and the sound was literally racing around the room, up the walls, across the ceiling and dropping in your lap.

*My ceilings are 9’ and I sit approximately 10 feet from a 4K 75” screen.

Wife's looking at me daft but we have flat 8 foot ceiling, just measured it . 

I've decided on the Sony HT-A9 in the end, seems the best option for our living room.

They are like rocking horse poo at the moment mind!.

Well this is coming from an old man so do with it as you want or need. I have never heard the new Sony system, but I’ll tell you this. Over my years I’ve owned a lot of different receivers and speakers including Klipsch and Polk & Bose which was the biggest of crap I ever owned. Then I bought the Sonos Arc, and the new Sub along with two One SLs for the surrounds. And I have neverhad such great sound in all my life. I’m using a new Sony OLED with eArc & Google TV, Apple TV 4K, Roku Ultra 4800X and a Fire Stick 4K Max and they will all play Dolby Atmos! Finest system I’ve ever had in my life. By the way with old Sony I had to use the Arcana but there were two many connections and always required fiddling around. If you can get yourself a new tv with eArc.