You don't say how old she is. Size isn't all that matters when deciding to put your child in a booster seat. They have to be mature enough to buckle and stay buckled, sit straight, keep the shoulder belt in the right place, etc.. My 6 yo and 3yo both ride in Britax Regent's - it has a 5pt harness that will take them until 80lbs or 53". This seat has two mesh pockets, one on either side that easily hold a cup and other stuff as well. My 3yo is almost 40" and weighs 38lbs, he is the smallest boy in his preschool class and close to your daughters size, I can't imagine putting him in a booster right now. Please choose wisely.
*By the way, I found both of my Regent's at Amazon.com and only paid $179.00 each (they may not be this cheap now, that was a sale price) but it's a small price to pay for peace of mind.
M.