I can speak to this a little since I currently own and play both...
I have a Besson 765 that I've been playing for about 25 years. At the time I bought it I couldn't afford a compensating horn and the Besson seemed like a good choice. It has been a good horn and I still use it regularly. It does however have some tuning quirks because it is non-compensating. Most of these I have been able to deal with over the years by "lipping" the notes up or down or using alternate fingerings. Recently, in a quest to find a good, inexpensive horn for my teenage son, I picked up a Wessex Dolce. This horn is also a good solid player and is a compensating horn, but still does have some tuning issues. I compared the two very closely and played both regularly for about a year and in the end I decided to go back to my Besson 765. Mainly because I like the horn, am comfortable on it, and liked the tone a little better. Personally I would say that the two are very closely equivalent horns, but for now I think I'm going to sell the Wessex and continue to play the Besson until I can get a Miraphone or Adams.
Good luck in your search. I don't think you will go wrong with either choice.
Doug Prowant
Former Band Director, Current IT Nerd
Besson New Standard
Boosey & Hawkes Imperial
Besson 765
Besson Brevete Baritone
Conn 88H Trombone
Gone but not forgotten:
King 2280S
Willson 2900S
Wessex Dolce