Every mattress store, it seems like, sells mattresses by a different 'name'. It makes it really difficult to compare apples to apples when comparing prices.
I would simply suggest this: do NOT get the cheapest mattress you find. Get one that has something to it. A $200 mattress will sleep like a $200 mattress. And then, after a year or two, it will sleep like a $50 mattress. Yes, she is small now, but when she is 6 yrs old, she won't be so small. Go for something higher quality.
We just bought a $500 set (full sized) for my daughter... "discounted" to $400. It is a FABULOUS mattress. The $300 set (twin) that she had before was fine for a 3 yr old (which is how old she was when we bought it). But when she started to grow, it didn't hold up. Yes, she ended up getting 7 years out of it---but we should have replaced it sooner than we did. If we had bought better quality to start with, she would still be sleeping on it. As it was, when we broke down and replaced it, it felt odd putting her back on a twin bed at 10 years old. So we bought an entire new BED, so that we could put her on a full. An extra $100 now will be worth a lot more than that in the wear throughout the life of the mattress. Trust me.