Is it better to do undergraduate research in the summer, or during the school year? Also, if an undergrad does research during the school year, are they expected to be as productive as someone working during the summer?  Do professors calibrate their opinions of students to account for how many hours a week they've said they could commit?
I think the main question is a false dichotomy. Most students don't have a choice between either doing research over the summer or doing it during the year. As long as it doesn't interfere with your grades or other activities, more research is always a good thing, no matter when it happens.

That said, in my experience you should simply have a conversation with your professor (or direct supervisor, who will often be a grad student or post-doc) about time commitment and expectations. No one expects you to do the same amount of work in 10-15 hrs/week that you might do in 30 hrs/week, and no one expects you to work full-time while taking a full course load. As with many things in life, just make sure to communicate about expectations.