IMHO the 610's are the better quality speaker regardless of the fact that they cost slightly less than the 751's which were overpriced anyway. Back in the 90's B&W had two very distinct lines of speakers (low-cost and pricey) which used different sets of drivers, and nothing in the mid-price category to compete with the likes of Wharfedale, Celestion, Mission and such. This was okay as that segment wasn't a large market back then but eventually the advent of mid-priced speakers of audiophile quality such as Paradigm, Heybrook, Monitor Audio, etc. prompted B&W to come up with something that was quite better than their low-cost offerings without costing an arm and a leg: the 600 series which is still B&W's best-seller.
The DM610/610i is an early member of this family, it's the direct ancestor of the popular DM602 series. The 610 is a large bookshelf speaker featuring BW's antiresonant synthetic molded front panel fitted on a well-braced 30-liter sealed cabinet. For a so-called bookshelf speaker that's about as large as it gets, and ideally it should be placed on a an 18-inch or so sturdy stand (they are quite heavy). I figure BW opted for a largish sealed cabinet and 8-inch mid-bass driver instead of a smaller vented cabinet/6-inch driver for better low-end control and ease of placement but I guess market response was leaning more towards smaller vented cabinets because subsequent editions of the design (rebaptised DM602) adopted this design.
The 610 sounds great in a mid-size room, provided that it is hooked to a decent quality amplifier (I liked them with Sugden but Arcam should do fine as well), clean tight bass response to about 45Hz, surprisingly clear mids for such a large driver (but don't expect Rega-like heavenly mids from an 8-inch driver) and the slightly restrained yet pleasant highs the well-designed BW tweeter of that vintage was famous for. They can be driven quite loud, enough for the physical effect of the bass to be felt without audible distortion or feeling of sound compression (dependeing on amp used obviously). At the time this speaker had no equal at its price point, and probably still doesn't.
I do not feel comfortable comparing Mission speakers to B&W 600 series and up, they are not in the same class. Don't be fooled by the modest price commanded by the 610 on the used market: it's its size and weight that make it less desirable than smaller bookshelf speakers, not sound quality. Personally I prefer the sound of the 610 to the later 600 series. I still listen to them quite often even though over the years I have acquired more sophisticated equipment. Matched with the right equipment they are still quite endearing