Delaying grad school vs. going to lower-ranked school
I don't entirely disagree with the two previous answers: taking the offer you have now may be a fine choice.  However, I'd give serious consideration to strengthening your application and trying again.

One of the biggest differences between departments is the strength of the peer group.  You will ultimately spend much more time talking and working with your fellow students than with faculty, and being part of an amazing cohort is incredibly beneficial.  You'll spend countless hours in detailed discussions, going into far more depth than any faculty member is likely to have time for, and you'll organize student seminars and study groups.  Depending on your field, you may write papers with your fellow students with no faculty involvement.  In the end, you'll learn far more from them than from courses or meetings with faculty, and these relationships will last for the rest of your career.

If you don't have this opportunity, you can certainly still become an excellent researcher, but you are less likely to reach your full potential, and even if you do it will be harder.  To maximize the benefit, you need to be surrounded by lots of wonderful students, many of whom are just as talented, hard working, and ambitious as you are, and some of whom are destined for even greater things.

As a rule of thumb, as you move down the prestige scale the average level of the students drops off much faster than the level of the faculty, because there are many more students than faculty.  There can be exceptions, such as a department that attracts much stronger students in one subfield than overall, but you shouldn't count on it without concrete evidence.

Here are a few ways you can gauge the students in a department:


When you visit, do you have really interesting discussions with current students, which leave you feeling excited about further interactions?  (Don't take this too seriously, since if you're shy it may be difficult to connect quickly, but it's worth thinking about.)
Does the department regularly produce graduates whose careers you would be happy with?  (Take this one very seriously.  If you are aiming for a job at a major research university but few people from your Ph.D. program get such jobs, then either your goals are unrealistic or you'll really stand out compared with your peers.)


If the answer to both questions is yes, then that's a good sign, but otherwise you should be cautious.

Returning to the original question, this might help you judge the offer you have already.  For the option of spending another year where you are, I'd ask three questions:


How likely are you to get better offers next year?  Of course there's no way to be sure, but perhaps a mentor could help you make an informed guess.
Will you enjoy the next year?  It's not worth doing if you're going to be stressed out and miserable.  Besides, unhappy people rarely do their best work.
Will you look back on it as a valuable intellectual experience, regardless of its effects on your grad school applications?