WOW, thats a loaded question....
Honestly I am not one to tell people what speakers to buy, that is a very personal choice. I am really someone that likes floorstanders, especially if its a large room. I'll give you some thoughts, but I wouldn't begin to tell you what speaker to buy... there are tons of speakers out there, and there are truly alot of choices at that price point these days.
All I can do is give you my opinion...... But I am not the best at this type of thing. So I will link you to Nuance's thread to let you read his description of both, he puts it really well into words, better then I. Also, he owns the 850's so he knows them very well. He is my neighbor, and has been over to my house a number of times, and the links I am giving you, are part of his " My Journey to find the "perfect" speaker..." Thread over at AVSforum.
First off, both speakers to me really sound fantastic.I don't think you could go wrong with either selection. They both would benefit from a good sub. All speakers were run full range at the GTG
This was the first time I have ever heard any of AV123s speakers, and to be honest, I wasn't expecting them to sound as good as they did, I was skeptical. But all bull$hit aside, they are very close to the sound of my Paradigm Studio 100's. Great speakers, full rich sound from top to bottom. I would be happy to own them, the finish was better then I expected as well... very very nice. Great sound stage.
Ascend Sierra 1's...
These little things surprised the hell out of me, they have an incredibly full rich sound, very smooth, very impressive. I couldn't believe the bass these things put out, truly had a great presence and wide, deep soundstage. To me they were worth every penny.
These are Nuance's direct reviews from my GTG
Ascend Sierra 1's
http://www.avsforum.com/avs-vb/showpost.php?p=13112928&postcount=622
SVS MTS', Onix Rocket 850 Signature's and Paradigm Studio 100 V4's (floor standing speakers)
http://www.avsforum.com/avs-vb/showpost.php?p=13168444&postcount=726