The newest versions of the button gauges are about 1" in diameter and are a lot more readable under water than the older 1/2" button gauges.
However, not all of them are created equal. Some have a scale that only travels about 120 degrees rather than 180 degrees so they offer significantly less definition and are still hard to read.
On a stage bottle I prefer to use a 2" SPG on a 6" or 7" hose since the 100 psi increments allow greater accuracy when turn pressures count.
On a deco bottle, I either use a button gauge or don't use one at all. Accuracy, beyond confriming it is full prior to the dive, is not important on a deco bottle. You plan your deco gas and then bring along 1.5 times what you need. Knowing the pressure during the dive does not really help as you can't make more once you start the dive. On the other hand, it is reassuring to have a ballpark indication, the button SPG is useful in terms of having a needle to watch during gas switches, and it is a good visual indicator to let you know the reg is still pressurized or depressurizing during the dive with the valve off.
Consequently I tend to compromise and use one of the larger and more readable button gauges on my deco bottles. It does not add as many failure points (no hose and no HP spool) as a regular SPG and it is quite streamlined.
Some divers feel it is important to set up your stage and deco regs so that they are identical. That makes sense if your regs often serve as both, but if they are dedicated to stage or deco roles, I prefer to optimize them for those roles and eliminate the hose and extra complexity on the deco regs.
With a pony the primary consideration needs to be bullet proof reliability adn if you are using it, it is because your primary failed and you either have enough gas in the pony or you don't so checking it is just wasting time and gas. But unless you take a pressure checking gauge along with you to check the tank on the surface before every dive, a larger button gauge is a good idea as it makes it easy to confirm the pony is still full prior to the dive.
The picture below should give you a good comparision. The one on the left is a 2" SP gauge on a 6" hose and the one on the right is a 1" OMS button gauge. There is some glare from the flash on the button gauge, but it is a pretty good simualtion of what can happen under water with the flat lens face. It is harder to see but still readble.
