I don't think you can get higher-end 18" bikes. They only come in 16" or 20". That will limit your options, if you decide to go with the 18". My personal preference would be to get a 16" bike from a good brand. I'd rather get a used high-quality 16" than a sparkly new low-end 16 or 18". A more expensive bike will be a lot lighter and easier to use, which will make it easier for her to learn to ride without training wheels. It'll also have better re-sell value, so that you can recoup some of the cost when she grows into a 20" bike.
It's easier to learn to ride on a smaller bike, though it's hard to tell which bike is the better fit for your daughter. It depends on her. My 4.5 year old rides a 16" specialized hotrock, and he still has plenty of growing space. We got it on Craig's list.