No one has actually heard what the Roam or a pair of Roam’s sound like yet, so any answer at this point would just be a guess.  I do have a Move and am very happy with it used outdoor, particular over WiFi when auto trueplay is active.  My guess is that 2 Roams will sound good and give you better stereo separation than a single Move (when over WiFi), but will lack the bass a Move can provide.  As well, Roam’s aren’t likely to be able to get as loud as a Move can get, so if you have a larger space, a Move may make more sense.

I have the ‘Move’ too and love that speaker, but I will also definitely go get a pair of Roams, as I can imagine they would be great stereo-paired nearby on a tabletop/desktop - I can see me taking a Roam in the car, or whilst out walking/camping too, whereas the Move is too large/heavy for general walkabout purposes - so room for both types of device, perhaps? It looks like the Roam currently has more features than the ‘Move’ too with Sound Swap! and Qi-charging.

I love my Move. It’s pretty big and heavy, but the auto-tune and the sound quality and battery life is a knock out. For my use in my backyard which is pretty big, I have decent wifi coverage. I like the look of the Roam, but I don’t really have a use case, for holidays or travel I have a pair of UE Boom’s that do a stereo pair via Bluetooth really well. 
 

For my average weekend outdoor use at home, the portability of the Move on wifi to match what’s playing  inside can’t be beat.

The Roam wouldn’t really replace my Move for my circumstances: at home, outside, on a wifi network, weatherproof.

I'm hooked on Sonos like many others. I too have the Move & pre-ordered 1 Roam. To answer your question, it depends on your expected usage.

Sound quality: my guess is the Move is 10x better than 2 Roams. (just a guess). 

Local portability around your property/deck: Move hands down. Sound is loud and has a good bass.  Remember it also has Bluetooth. 

Vacation/trips/hiking, etc: Roam. That's what it's designed for. We're all assuming it'll be smaller sound from a small device.

So if 75% of your usage is home/property. Go with Move. If you want 2 Roams for trips then great!

Personally, $340 for 2 roams is NOT worth the money over 1 Move for $400 for sound experience.  I know it's a lot of $ but if over the next year you can splurge, get a Move now, then later get a single Roam. 

THIS.

Plus you can’t stereo pair two of these Roams on Bluetooth, only on your home wifi. The big sound from a Move is going to be very hard to beat. Average home wifi should have you covered in your yard, and being able to have the same music playing from one end of your home to the other, indoors and out is (for most of us) the reason to invest in the Sonos devices to start with.