The floating glass hydrometers are, obviously, more fragile. They are also a bit more trouble, in that you must either shut off any strong water circulation if the reading is taken in the tank itself, or measure the water in a container deep enough to float the hydrometer. Many hydrometers come with a round plastic column designed to hold the sample water.
If a new glass hydrometer is $4, something is wrong. I have seen tiny cheap hydrometers that are about 5" long. This is not a good hydrometer. A real hydrometer should be about 9 to12 inches long. They used to be carried by all serious marine aquarium stores, but have been supplanted by the plastic gizmos, which are cheaper to make, smaller and cheaper to ship, don't break easily, and earn higher profits. They can give a ballpark figure, but are very imprecise, and affected by dried salt, micro debris, air bubbles, etc.
The best hydrometers I've seen are from Germany, where aquarium technology and quality are far beyond standards prevalent in the United States. They are willing to take the time and spend the money. Few of us are, so a lot of junk is marketed.
If your water temp is 81F and the hydrometer is stanardized in the mid-70s, there is no significant difference. If the hydrometer is standardized in the 60s, your water will be a little more saline than the hydrometer reads: a reading of 1.020 on a 60s adjusted
hydrometer in an 80s tank means the water is really close to 1.025.
If you are in the very broad salinity ballpark, your fish will be fine, at least in terms of salinity issues. Most systemic problems that I personally have seen in the SW tanks of the inexperienced are caused by inadequate water circulation. Strong water movement in all parts of the tank is critical in SW aquariums, far more than in FW tanks.
