Both are idol animes about working in an industry.
-Have a variety of idol (inside and outside anime)
-They experienced similar obstacles (Jupiter with 765 Pro & Trigger with Yaotome Pro)
-They grow closer as a group as each episode passed by
Both stories are music anime which focus on male idols and have comedy elements.
The main differences are that in UtaPri the main character is female, while in IMas the characters are all males (there are no romance elements); and in UtaPri the idols start their music experience at school, while in IMas the characters are directly employed by the music agency.
If you like funny, colorful music anime with male idols, you'll probaly enjoy both these series.
Both feature upcoming idols from different groups. The stories also have a slice-of-life take on the lives of the idols, showing their interactions not only with others, but also with the idol world. The music for both is also pretty good.
Plucky amateurs struggle to become famous idols so they can bring joy to everyone in fun character-focused dramedies. Much more focus is placed on the idols themselves and their relationships with each other than providing any kind of realistic look at the industry.