There are some episodes centered on the very thing. VOY often runs into alien AIs and the plots sometimes revolve around their rights or lack thereof. In VOY "Author, Author", the ship's holodoctor (or Emergency Medical Hologram) starts writing polemic fiction on the treatment of holograms, and the episode concludes with a view of the Federation apparently employing sentient EMH programs in menial mining duties but these starting to foment rebellion after gaining access to the Voyager EMH's writings. This might indicate there are moral questions to be addressed, of the exact sort brought up in "Measure of a Man". OTOH, since much of the episode involves the fictional viewpoint of the EMH, we can also fairly conclude that the concept of fellow EMHs toiling in mines and being liberated by the hero EMH's writings is utter fiction as well...
We do know that holograms are seldom considered to be sentient and treated with appropriate respect - even though much of holoentertainment is based on the holocharacters convincingly faking sentience. The big moral question thus seems to be whether faking of sentience is grounds for respect or not, and whether faking can be told apart from the putative real thing. Should we politely say hello to window dressing dummies because they look like us, just in case? This issue is implicit in the holodeck-heavy stories of VOY, but never really explicit. Our heroes treat characters as sentient within the context of a program, but simply turn them off (or turn them into weapons) when more pressing matters arise. They do the same to the EMH, who sometimes is miffed. Or does he only fake being miffed?
It is rather logical for Data and the holograms to be viewed so differently as regards their (possibly faked) humanness. Data is a unique miracle of technology; holograms are household items, and have been that at least since Janeway's youth.
the "sentient hologram" storyline always seemed a bit silly to me. Data was at least built to be a self-aware being who could make his own choices. Holograms are totally programmed characters designed from a computer. It would be like a video game character becoming self-aware. If you played "arkham city" long enough does Batman become self-aware?
Sure, he would. Cause he's BATMAN!
Seriously, though, it's rather a blurry line as to what makes a hologram sentient. Is it simply the knowlegde that they ARE a hologram. As seen in "The Big Goodbye", seemingly any hologram can learn that.