Hi there!
That's a tough one because she's right on the cusp. If she were a year younger, I'd recommend getting another harnessed seat. If she were a year older, I'd recommend a booster. At her age, she's sort of right in the middle.
Personally, I like to see kids harnessed until 6, and it's usually better for them to be at least 40 lbs. You might want to consider a combination seat: One that only goes forward-facing with a harness, then turns into a booster seat. That would let you get her until at least 6 and 40 lbs (or well beyond, if you want), and then you have it as a booster later. Here's a list of CarseatBlog's recommended convertible seats:
http://carseatblog.com/safest-recommended-car-seats/#COMBINATION
On the other hand, as long as she meets the minimums for booster use, and she fits properly and will sit properly the whole time (not all 5-year-olds can), a booster is also a reasonable choice.