I know that these are obviously both bad and would presumably fall under gilui arayos, but which is worse?
Based on my limited understanding of the halachot, if we are talking about adults, then it is a mitzva d'oraita1 to not gaze at them, whether it be lustfully or simply to enjoy their beauty. It doesn't matter if they are Jewish or not Jewish2.
If they are under the age of 11 then we find a difference in halacha. By girls under the age of 11 who are Jewish, one would be permitted to enjoy her beauty (but obviously not have lustful thoughts), whereas by a non-Jewish girl, this would still be forbidden as she is still considered an "erva" i.e. someone he is not permitted to marry.2
In general, one has other mitzvot and obligations towards a Jewess that one wouldn't have towards a non-Jew, including the mitzva of Ahavat Yisrael, Lifnei Iver etc. By gazing at a woman, one is being disrespectful, as well as causing a challenge in heaven against her. One is increasing promiscuity in our nation.
It's therefore not clear if it is worse or better, one shouldn't worry too much about that. One should make every effort to guard one's eyes. By doing so, one will bring bracha to himself and the world, instead of curses, and the reward is very great.
1 - See this answer for sources and details.
2 - There is a concept of "erva" by non-Jews, see SH EH 21:1
3 - See footnote 13 of pp.22 in Lift Up Your Eyes by R' S Wagshal for full discussion and explanation