Dual tubes allow depth perception.

I would take lower end dual tubes over a high end pvs14. I have like a 2700+ FOM no spot L3 wp pvs14 and a set of lower spec DTNVS with L3 tubes and you get so much more information from dual tubes and depth perception. I never use the pvs 14.

look for maybe a lower cost photonis setup in an articulating housing if price is an issue and you'll be amazed at how much better it is. I'd take dual photonis tubes over my high spec PVS14 any day.

You could also join the thermal master race and run a bridged pvs14 and a RH25 or some other thermal monocular. Don't overlook that option.

I mostly agree with Will. Dual tubes don't give any extra peripheral vision; what they give you is stereoscopic vision with both eyes seeing the world as they normally do. This helps tremendously in situations requiring fast decision-making since your eyes aren't competing against each other, and your brain doesn't have to spend time rectifying the two competing images.

Personally, I don't find the trade-off for using a mono in mixed lighting worth it; I just run my binos far out on the mount and look under them as necessary. That way both eyes are always seeing the same thing.

My only preference for monos is when form factor (size and weight) is a factor while decision-making isn't. Examples for me would be backpacking or stargazing with the family. For anything serious, I prefer binos.

Your brain does a better job deciphering information with both eyes working together. The "percieved" depth perception is much better with duals.

The only real negative to duals over a single is weight, and ability to gauge ambient light conditions. A single 14 is also handy for photography of you do any astronomy. Easy to put into a tripod.

Wanted to add, if not too late.  After the initial novelty wore off, I didn't really "see" much difference between the two.  In my case I do long slow walks through the woods.  I actually prefer a mono for this.  I like having one un-aided eye.  This could just totally be me (and might be judging from other comments).  

I would also add, I like to run thermal as much as I can, in an articulating bridge with a -14.  Having this kind of dual spectrum capability is the next best thing to the newer fusion stuff.  So a good -14 upgrade for me would be getting a really nice L3 Unfilmed tube.  

If any of this applies to you, then for your consideration.