I most favor an old fashion analog (needle) type voltage gauge as they are the easiest to read for those of us with eyes that don't do well a close distance anymore and they most resemble the stock gauges. Look at
www.electricalconnection.com I have the larger 2.5" gauge installed in the upper left hand corner of the cowling above the storage pocket/bin and if it did not say HONDA you would swear that it came straight from the factory built right in; if I had not already had a larger hole in the dash from a red LED gauge that drove me nutz as it was to hard to see without glasses on and flickered the last digit too much I would have selected the smaller of the two gauges as it would look spot on with the fuel and temp gauges.