The Samsung Omnia 7 is my first choice as well. I've bought a lot of non-US unlocked phones over the years but now they're too cost prohibitive and carrier support isn't as simple, so it looks like the Omnia is off my list.
I would recommend the Samsung Focus off the bat because I hate the Surround's design but since you actually like the speakers on the Surround that complicates things.
Screen
As you already pointed out, the Focus is basically the wp7 version of the Captivate. It has the same gorgeous 4" Super AMOLED screen.
The Surround has a 3.8 lcd lcd.
Apparently, wp7's ui means that the smaller screen isn't a substantial drawback.
Design
Mostly a matter of personal preference but here are some differences:
The Focus actually varies from the Captivate quite a bit design-wise. I personally prefer the Captivate's overall design. The Focus has a "lip" (don't know how else to phrase it) on the front and a curved top and bottom, compared to the angular design of the Captivate and Omnia.
The back of the Focus
has a subtle pinstripe print and a slight curvature. I'm afraid that the slick plastic of the Focus would be more prone to slipping out my hand than the back of the Captivate or HTC Surround.
The Focus is also the thinnest windows phone, while the surround has extra thickness and heft. partly due to the slide-out speaker.
The Focus's micro usb jack is located on the top and is protected by a sliding door; the Surround's is on the bottom and is exposed.
Build Quality
The Focus is plastic with chrome-like accents on the sides.
The Surround has a metal frame and a soft touch back.
The general consensus is that the Surround feels like a solid, premium phone and has a slight edge over the Focus in quality.
Camera
Both have 5 megapixel cameras with 720p "hd" recording; the focus has regular flash(?), while the surround has led flash. The focus has more camera controls (contrast, saturation, and metering) than the Surround.
Based on the reviews I've read, the Focus's camera is better than the Surround's for stills and video.
Samsung Focus Cnet photo sample
Surround Cnet photo sample
Mobile Burn Surround HD Video Sample
Samsung Focus HD Sample Wp7 does 24 frames per second
Speaker
The Focus has a traditional speakerphone. According to Cnet:
Though not as loud [as the earpiece], the speakerphone provided enough volume that we could carry on conversations in a noisier environment. The sound quality was slightly hollow on our end, and callers said there was a bit of an echo on their side but nothing that prevented us from carrying on with the conversation.
The Surround has a slide-out speaker. According to Cnet:
The speakers actually offer surround sound via Dolby Mobile and SRS WOW HD and to activate it, you just press the small button on the left-hand side of the speaker.
There's a noticeable difference when you do this, as the audio sounds richer and fuller. However, the overall sound quality didn't really knock our socks off. It's certainly better than most, but songs still sounded somewhat tinny. Also, during a speakerphone call, the volume, even at its highest level, was too low to have a conversation in a slightly noisier environment. For being the phone's highlighted feature, it's certainly not the selling point for the phone. We already felt like the speakers were bit of a superfluous item that would only appeal to a limited number of people, but now even more so.
Apps
The Focus comes with a "Now" app. It includes weather, news, and stocks. There is also a Samsung Zone in the maretplace that offers "network profile", "3" and "photo sharing". Haven't seen any additional info on these three.
The Surround comes with the HTC hub, which features a Sense-like clock-and-weather screen, along with other simple apps - Converter, Photo Enhancer, Sound Enhancer, Stock Ticker, Notes. There are some videos of these apps on YouTube.
Storage
The Focus has 8 gb storage on-board. Storage can be expanded up to 40 gb total, via a class 6 micro sd.
The Surround has 16 gb storage on-board. Not expandable.
Battery Life
The Surround fares much better than the Focus in everything except 3g talk time, according to
AnandTech. Actually, the Focus's battery life is quite poor. There's always the option of carrying an additional battery or using an extended one.
I *think* that covers everything. Both phones have their strengths so it's really a matter of your needs and taste. I would definitely encourage to get hands on with both phones in person. You can even try both out for a while within the 30 day exchange period.
Here are some more reviews:
ZDNet comparison
Focus
Cnet
Anandtech
Mobile Burn
Engadget
Phone Scoop
Surround
Anandtech
Mobile Burn
Engadget
Phone Scoop
There are also several in-depth review videos on YouTube.