So after spending the weekend with my new 50s and 28s, I've been thinking about what to say that will add constructively to this discussion. I guess it comes down to with the budget and enough space, and if you really enjoy critical listening of music in addition to watching TV/movies, I'd definitely go with 50s/18s over 2 pairs of 28s. If you don't have the space (considering you'll need at least 5.5 inches on either side of the Harmony for the speakers to open up without hitting the 50s) or care about more casual listening, or just don't want to spend the extra money, then I think the 28s would be a great option.
The 28s are nice speakers, with quite an impressive soundstage. Upper frequencies and midrange are amazingly clear and bass is strong and is tightly controlled after calibration on most music and at most listening levels. To my ears the bass is still a bit overbearing on some music and at higher volumes. It's not displeasing, it's just not what I'm used to from B&O. It's a different sound signature.
I tend to prefer music and movies that are as true to the original intent as possible--you could read this as "neutral"--and the bass on the 28s on some material and at higher volumes gives music a different and seemingly less neutral feel. I'm happy I got the 28s--and they work well for my office where I placed them--but I'm also happy that I went with the 50s for my main room to pair with my 18s as rear surrounds. I wanted "reference" speakers for my main listening area, and I think the 50s fit the bill there in a way the 28s simply cannot.
My time with the 50s has been nothing less than sheer joy. Given the size, design and seating constraints of my living room, the 50s aren't in the "perfect" position (nor are they toed in), but it doesn't seem to matter. In the short time I've had them I've heard nuances in songs I've listened to hundreds of times before that I could not previously discern. There's ample bass that sounds natural and effortless, and regardless of what type of music I throw at them, they sound phenomenal. (So far my favorite has been Solsbury Hill on 45rpm half-speed remastered vinyl--utterly amazing). I had high hopes for them, but they've surpassed every expectation. The best way I can describe it is that there's a fullness and richness to the sound that it's like I'm inside the music.
I don't know if any of this is helpful to you, but I wish you luck with your decision. It's all great equipment, it's about finding what's best for you. Good luck and keep us posted....