I've owned all three.
The CTX has been my "go to" simply because I have more experience with it. I have lots of experience with the Deus, but I liked the ruggedness of the CTX more and the display. The Deus cannot be beat if you want a compact and light unit that will stand up to anything out there. The Deus sounds especially in multi tone are a bit high pitched and different than anything I have heard. I do think you can adjust them. The Deus is so fast especially if you raise the reactivity that you have to listen well for the short, almost clipped tones that it produces.
The V3I is a great detector. My only true complaint is that it would have seemed that White's could have done a total redesign, but if it works why change it, I guess?
Also, the V3I has a lot more variables to tweak then the CTX. I think that someone could get VERY comfortable with all the settings on the CTX much faster then the V3i. The V3I display is also top notch, and there are a lot more (and affordable) coils out there for it.
If money wasn't a factor, I would have a V3I and a CTX. Learn them both and I bet you can pull anything and everything out of a site.
To me, all three detectors are very different from one another. It is very hard for me to determine which one to choose if you had only one choice. Something to consider also if money was an object is to throw an Etrac into the mix with the V3I and Deus. True the Etrac is very different from the CTX, but I feel performance wise they are close.
Best choice, probably would be to get one and learn the heck out of it, and then acquire another. It is hard to learn multiple machines at once. Anytime I switch it up, I have to spend the first 20-30 minutes of a hunt wrapping my head around a different machine before I start to really feel comfortable.
As a general rule, I think if it is at all possible swing a detector on an actual short hunt before you buy. I've found that 90% of the people using a certain detector will love it and then 10% will hate the sounds or the ergonomics or something. I know that I am that way. Then you have to spend your time, adjusting to the detector instead of the detector adjusting to you. This can also vary by models or even brands. I have swung a lot of White's and Minelab and they feel and sound good to me as a whole. The Deus is way different and honestly, I just didn't completely feel it, but I do miss the ability to swing it all day. It is like having a stick in your hand.