One of my professors from undergrad would argue the opposing view point. His opinion, and that of his colleagues, he stated, is that one summer is superior to two (academic) years. The reason is that during the school year, assuming an otherwise full course-load, the most undergraduates can typically commit 12 - 16 hours per week in the lab. To the contrary, during a summer project, the undergraduate can commit 40 - 60+ hours per week.
Before I say anything that you might consider, just wanted to let you know I am in the spring semester of my 2nd year at a state school. I am just an undergraduate student.
I really got involved in research the summer between my first and second years.
I have continued studying in this lab and doing my experiments throughout this entire academic year. I am planning on coming back this summer, and continuing through my 3rd year as well.
I have to say that I accomplish the same if not more during a full summer spent doing research versus an academic year. Not to mention (I hope MD/PhD ADCOM looks at this when evaluating students) when you do research over the summer it's all about volunteering. I didn't do it for credit last summer, and this summer my PI asked me if I wanted it for credit. Seems meaningless to me to pay upwards $1100 for 4 credits of research experience when I value volunteer experience as just as satisfactory (can someone please let me know if ADCOM cares whether you get credit or not? I would expect they value results and what you did during that research experience more). I also have to find summer housing (ya, it's cheaper than during the year... but spend 3-4 summers and it adds up). Therefore I'm taking a lot out of my pocket and am not getting a lot out (other than research).
This being said, I think the ADCOM should look at it very positively when they see a student doing research in a lab over the summer. However to look at it negatively why the student did not is harsh... I think not everyone can handle the financial burdens...
Should a student be lucky enough to work in the same lab as he did during the school year, +1.
If not, and the student is unable to, but still manages to do some work at a local nearby university, +1 again.
tl;dr: If you're lucky enough to be able to work in a lab in the summer full time, I would consider it equal to at least half a year. It is lacking in the sense that there is no longevity in your work. However it is superior in the sense that you get a lot of work done.