Yes, Mustangs and Sunbursts are indeed the same species (Hippocampus erectus). As such, they have identical aquarium requirements and will interbreed freely. They differ primarily in their coloration: Mustangs tend to be darker colored, displaying the dominant dark brown to black coloration that is so typical of wild erectus, whereas the Sunbursts tend to be more brightly colored, and typically display the much less common yellow to orange color morphs. The price reflects the fact that the Mustangs display the normal coloration for H. erectus, whereas the yellow/orange specimens represent a much rarer color type.
But it’s important to note that the Sunbursts are not genetic mutations that are locked into specific colors. Colorful Ocean Riders are not homozygous recessives nor or they mutations that are unable to manufacture certain pigments altogether. In other words, they are not like albinos that are always white because they lack the ability to produce any sort of pigment, nor are they like lutino mutations that are always yellow because they lack the ability to manufacture any pigments other than yellow. But they do exhibit differential proliferation of chromatophores and this gives each type a predisposition to display certain colors.
Mustangs, for example, have a preponderance of melanophores (black pigment cells) and tend to be dark (earth tones) or cryptically colored most of the time. But ‘stangs also have bright pigment cells and they can brighten up when the occasion calls for it, such as during courtship or when competing for mates.
Although yellow and orange pigments tend to predominate in Sunbursts, they are equipped with a full range of chromatophores and can display a wide range of colors. This means they are predisposed towards the sunset colors (yellow, gold, peach and orange) when conditions are to their liking. However, they have a complement of melanophores in addition to their bright pigment cells and are able to change their coloration to reflect changing circumstances and conditions. So yellow and orange are the most commonly seen colors in Sunbursts, but you also find them in white, pearly, tan or even brown color phases from time to time.
In short, Mustangs tend to be darker colored as a rule, but will show brighter color phases from time to time. Likewise, Sunbursts tend to exhibit the sunset colors when conditions are favorable, but they also display darker color phases on occasion.
You can expect both Mustangs and Sunbursts to go through quite a range of color phases from month to month. For example, I have watched my pair of Mustangs go through a number of color changes over the years. One has settled on a dark grayish -green as its base coloration for the moment, and the other ranges between rust, burnt umber, chocolate Brown and russet-brown, but always with contrasting beige bands. Last season, the male adopted a rich ochre yellow as his everyday attire (still with the same beige saddles, though), while the female displayed a dark purplish ensemble with definite greenish highlights. When courting, they consistently brighten to a pearly white and a creamy yellow respectively. They make a handsome couple, and I find my Mustangs to be very attractive specimens in all their guises.
Best wishes with all of your fishes, Curio!