I had the same deliemma... I got a killer deal on a sunpro 5" phantom tach, and I declined cause I already had the tach in the cluster.
In my eyes, the best compliment is when I can show a person my car, and have them say:
"So what did you do to it?"
The cleaner the install, the more "factory" you can get it to look, the better imo. I don´t like the look of cars with aspects that don´t fit the rest of the car, so example, painting only the gauge section of the dash a bright blue when there is no blue in the rest of the interior or exterior... it just rices out your car.
I would suggest possibly getting white gauge faces in place of the stock ones. A clean install will look factory and enhance the look of the interior, without having large tachometers all over the dash, and having to cut holes in the hood.
My 2 cents.