With the Arc SL age, I would opt for the Ultra. If you have used the SL without a Sub and surrounds, the Ultra is going to make a huge difference for you.

If you do not have another location for the SL, you can always sell it in order to get surrounds later.

Do you have existing surround speakers or a sub?  I can;t speak to the difference between an Arc and Arc Ultra, but I would likely look into completing the home theatre setup before upgrading the soundbar or other part of  my home theatre.

None of your options…save your money till Sonos gets its act together. 

​@GregGizmo 

I also would go with the Arc Ultra as the up to date generation. It really is a huge difference to Arc (SL) in single use mode, especially regarding to bass and voice clarity. 
 

Arc Ultra is a little too buggy for my taste. Arc, 300s, and a sub provides fantastic sound. Enough for me not to want to upgrade.

Just out of curiosity...what bugs are you experiencing with the Arc Ultra?

Several posts on arc ultra issues in the fb groups..not sure exactly. 

​@GregGizmo 

Just so you know the Arc Ultra has microphones built-in for Voice assistants. They can be silenced via a security switch on the rear.  I only mention this as most buyers of the Arc SL did so because they wanted no microphones whatsoever in the Arc. 

I don’t know what the FB is??? However, my question was to ​@GC3 as the OP spoke as if they have hands on experience. BTW...I have three (3) Arc Ultra’s and have not noticed any bugs.

I didn't buy one. I'm not experiencing any bugs. However I have seen posts either on this site or on the primary sonos Facebook community about others who are experiencing volume-related issues. I consider this useful information.

Hmmm… I have three (3) Arc Ultras with no such issues. Besides I find most information on Facebook very suspect. However if you consider Facebook comments on the Arc Ultra useful information… then each to their own. I’ll rely on my personal experience to critique the Arc Ultra’s performance.

I have the Arc Ultra myself and it's a fantastic soundbar.

Of course it would be optimal with surround speakers and a Sub.
But if you have to choose, the Ultra actually manages to give you the experience of surround and with its built-in "mini sub" you also get a very reasonable bass.

In my opinion, I would go for the Ultra and then sell my Arc, save up a little and over time buy the Era 300 and a Sub.

P.s
I have never experienced any challenges with my Arc Ultra.