I have recently been doing some experiments with various brands of watercolors to determine what i like and want to use on a regular basis. I have mixed most brands together including Holbein Gouache with Water colors. I have not tried Dr Martins but i love their inks!! I mix pan colors with tube and make pan colors out of my favorite tube colors. I tend to push the limits a bit. Price does not mean quality....
Here are my 2 cents. Windsor & Newton and Daniel Smith were more permanent when dry. M Graham, Lucas, QoR, and Utrecht could be re-wet and moved around. All are nicely pigmented and all worked well together and with pan watercolors, inks and high flow acrylics. The only brand I had an issue with was Sennelier, the color felt thick and sticky. Also, the pigment separated from the carrier and I could not get it mixed back up without emptying a lot of the tube. I had a few tubes turn out this way and the amount I squeezed out on to the pallet never really dried. That is concerning to me. Maybe the tubes were old when i bought them but my 10 year old W&N tubes never did this.
I suggest that you pick a common color and buy that color in each of the brands you want to explore, that way you are not investing too much money and you can see how each brand performs. I know that each color has different qualities but you will feel more confident in deciding which brand you like.