The six character - three men - three women structure is so familiar on sitcoms, that it is a little unfair to use it as a reason that Coupling and Friends aahe characters seem to spend most of their waking hours talking about their love lives. Aside from that, the biggest element of each show, at least at the start, is the central romance of two characters and how it affects the people around them.
Steve/Susan vs Ross/Rachel
Of course, the premiere couple on Friends was Ross and Rachel who spent a great chunk of the series apart. Coupling put their couple together and left them there. You might say that Friends thinks love and romance are about getting into the relationship but Coupling thinks love and romance are about being in the relationship. On the whole, Ross/Rachel were more of a fairy tale romance wwith Ross winning over the girl of his dreams who "discovered" that her true love was there all the time. Steve and Susan were far more pragmatic; there were no bells ringing out and dramatic audience "aw!" when they made it official.
I wasn't exactly devestated when Ross and Rachel broke up. For one thing, they'd been playing the unreasonable jealousy from Ross long enough to make him a complete tool. And they pulled out the help I can't balance my career with my family crap. (The Meatloaf Theory. As in, "You were late to dinner and now the meatloaf is ruined." Though how you ruin meatloaf I cannot imagine.) Getting them together was standard sitcom fare; so was breaking them up. On the other hand, the actual episode where they broke up is stellar and one of my all time favorites. They do a great job of balancing the comedy and the drama on a level that Coupling never even tried to do. Ross and Rachel tear into each other. Steve and Susan just sort of snipe a bit.
Steve and Susan's breakup felt like it was tacked on to generate a bit of suspense between seasons. On the plus side, the rest of the episode "The End of the Line" was fantastic. Pure farce, building to a climax complete with an ending full of doors slamming open as the entire cast winds up piling into the living room Noises Off style. Really though, it was less of a breakup and more of a quarrel that ended with Susan walking out.
And the reconcliation wasn't as strong. Susan just sort of folded and I was never sure what her epiphany regarding Steve was aside from the fact that he left cute drunken voicemail messages for her. Not that we needed to wallow in melodrama, but if you are going to go there, then pay it off when you do. "The End of the Line" has humor, style (the time shifting), and emotional resonance. "Split" - to me - lacked the sharp humor, didn't work the gimmick as well as it could have, and squandered the emotions.
On the other other hand, Friends dragged Ross and Rachel out for years. Years, people! By the time they wandered back towards each other, I had long since stopped caring. And the surprise baby did not help.
Sally/Patrick vs Monica/Chandler
I thought Monica/Chandler was more of an honest surprise when they pulled it out. Sally/Patrick was kind of coming from the beginning and they had tried to hook up previously with mixed results.
Monica and Chandler have more depth to the relationship, but that is mostly because they have more time and focus. In a way, it might be better to compare them with Steve/Susan, because both couples get together and stay rather than playing out the will they/won't they of Sally/Patrick and Ross/Rachel. But this is already horrifically long, so I will let you guys fill that in.
Chandler doesn't quite match up to Patrick, of course, who maps closer to Joey. (Or to toss out something else I won't follow up on, Ross kind of gives off that Smug!Patrick vibe sometimes, doesn't he?) Chandler and his fumbling really is more of a Steve. Ah, the fumbling. The need to have the most basic of things explained. The reliance on friend's perspectives to tell them what they thought. And - of course - the redemption through the management of a good woman.
Monica, however, is definitely a Sally in the anxious personality and determination to get a man. Of the women, Monica was the out-there-looking character, complete with self-esteem issues with her weight (like Sally's worries about her age).
Jane vs Phoebe
Phoebe is quieter in her oddness and it is a less self-serving kind of weird. Jane's insistence that other people rearrange their lives to accomodate her delusions is her defining characteristic and they often take it farther than is realistic. Self-absorbed is one thing, but at a "real people don't act like that" level, she'd have been ditched or arrested long ago if she'd done all those things.
Both characters occasionally get left out and have to have external things going on to give them story. Phoebe's family gives her the most definition, something Jane lacks. Phoebe also gets the only permanent external love interest (a new character, not coming from within the group). New guys don't last long on sitcoms. They change the sit. On the other hand, for sheer "Oh no you didn't," nothing tops Jane.
Jeff vs Joey
They don't really match, do they? Apart from being the only ones left, there's really no connection there.
Joey is like Patrick in that one of the main character traits is his endless success with women. Joey is nicer than Patrick, with more of a dimness to Patrick's disdain for mere mortals and their subconscious. Joey also seems less of a goal keeper than Patrick, who struck me as scoring women out of habit or boredom or because that's what he thought people expected of him. There didn't appear to be a genuine interest in the women or in the sex for that matter. Joey enjoys his hookups. Patrick enjoys talking about them to his friends.
When you get down to it, Jeff stands alone. He's a little Chandler (hapless) and little Phoebe (odd one out, has all these theories about life), but something all his own because of the sheer depth and inventiveness of his self-sabatoge and imagination.
Next up: Coupling's big switch: Jeff vs Oliver.