I prefer the straight 30# boost gauge. I like the fact that you have 30 graduations over 270 degrees of sweep as opposed to 30 graduations over 135 degrees (1/2 of 270 degrees boost/vac), more space between the numbers. You can detect the difference between 21 and 23# of boost easier. The difference between 21 and 23# of boost may be 1/16" on a boost/vac gauge as opposed to 1/8" on a boost gauge.
If I need a vacuum gauge for occasional testing, I can easily hook one up as needed. How often do you really need a vac gauge?
Go to your local speed shop and look at the different gauges to see what appeals to you. That's all that really matters, what you like.