I have owned and used both. I like them both too, but there are better detectors out there for not much more money. Since you have to buy a used or old stock Vaquero or MX5 with little or no warranty, I don't really see the point.
Anyway, just like thump7 said the Vaquero is about as good a Tesoro as you can get in my opinion. There are more expensive ones with more features, but the Vaquero will go just as deep and has excellent weight, balance, target separation and functionality. It will incorrectly identify deeper targets but does a great job down to whatever is the dimensions of the search coil used. Some on the fly knob turning is required sometimes so you thumbs will get a workout.
The MX5 is much heavier, slower in recover and I did not like how it would give a low threshold response on targets that were just out of detection range. It was almost like hunting in a mixed mode and made the tones go high-low on deeper targets. which was annoying. Otherwise, it has great features and will go very deep. Some people really liked the MX5. Personally I liked the MX7 a bit better but did not like the relic mode for the same mixed mode high/low tones confusion.