So, when you say Osborne, which model do you mean? I guess I'll assume that you mean the 94x series... Also, I've never handled the Manix, so take my words with that grain of salt.
For ergonomics, I think all three are great, though each will be different. The 94x has great ergos for light/detailed/fine/etc. work. The other two seem to excel more where hard use, hard gripping of the knife may be involved. That is, they'll fill the hand better than the Osborne, which is a plus for hard use. From your description, I'm going to venture a guess that either the Manix or Rukus would suit you best here. Between those, I'd probably lean towards the Manix.
For the blade, the 94x may not be the best suited if you expect you'll be using your knife as not a knife. If you are indeed using it for cutting, though, any of the three will serve you great. I think the Manix would be my choice for blade grind and geometry, but I would likely go with the Rukus if it came to fixed blade replacement type of hard use.
Fit and finish is likely to be comparable and excellent on any of the choices, so no need to worry there. For materials, I guess it depends whether you prefer G10/micarta, or aluminum. My instinct is to go with one of the G10/micarta models, though I don't have any good reason to back that up. Aluminum is excellent stuff, too.
For me, though, lock type is a very important consideration. And for me, that means no back locks. They just don't feel right to me, and I always feel like other lock types permit better one-handed closing. So if it were me, I'd go for one of the Axis models. And between the 94x and the Rukus, the 94x definitely has the easier-to-manipulate lock, and so it would be my pick here.
In the end, I expect either the Manix or the Rukus could fill the spot you're looking for perfectly. While I might lean towards the Rukus as a fixed blade replacement, as you said you were looking for, I think it may be wiser, as an above post stated, to go for the Manix. Better to buy a discontinued knife early, then get the ones still in production later.