The old fashioned dial types are best and more accurste than those plastic mercury ones. Either will do of course and you might just want to save money and buy the cheapest set. As the 1500 only had two carbs you only need a twin set.
I have both types, but, I find that the mercury type is more accurate because there is no mechanism to go out of calibration. The drawback to the mercury setup is that if you rev the engine and let it drop quickly, you'll have to spend more cash on replacement mercury.
The gl1500 carbs apparently need balancing every 5000 miles (I think the older Wings do as well). So maybe a mid priced set will last you years. The dial types are favoured by mechanics as there is no mercury to worry about and they are more reliable than you might think. That's from the few mechanics I talked to.
I've got the 4 analog dial set up and have been using it since Ben Hur was a baby.. Absolutely no problems and I think the set only sells for around $49.00 US. I believe quality in tools makes a good job a lasting one.
The only problem with the dial types is that you must check their calibration by using one cylinder as a vacuum source and testing each individual gauge on this one cylinder, then you'll know the gauges are equal, if not, you'll never get a good synch job from them. What happens with the dial type is that the internal bellows actually bends and reforms itself according to age, temperature and pressure.
The mercury type always has balanced atmospheric pressure and liquid metal of equal mass/gravity to keep it more accurate. (It's just a pain if you're not careful and snap the throttle shut too fast(high vacuum conditionand draw all the mercury into the engine.)
Actually, on a 1500 you could use one single dial gauge (or mercury stick gauge) because it's a simple matter of going back and forth from one carb to the other. Simply record the reading and make them match as closely as possible.
There are pros and cons to both types. I've heard of the mercury getting sucked out of that type, but the dial ones can get damabed as well. I have a set (although I never used them myself) and the instructions say that if you accidentally turn the choke on with the gauges attached you may damage them. So it looks like you need to be careful whichever set you buy.
They are real easy to setup and a complete novice could hook em up and balance a set of 1200 carbs in 30 minutes. No nasty mercury to worry about. The unit works on a 1500 valkyrie, but does not work on the 1500 goldwing too much vacuum apparently...