Several advantages of getting a higher end board. The main issue is board performance. A beginner has less ability to compensate for deficiencies in equipment and therefore, having good quality equipment makes the learning curve easier and more fun. The properties that make boards higher quality are things like the materials used in the board`s core that give it "pop" or spring as well as stability especially torsional stability (the board resists unwanted twisting). Over the long haul, as the rider progresses, a higher quality board will still perform to their increased demands, whereas a budget, beginner board will not. In the end, they spend more money by going cheap in the beginning and then upgrading equipment later.
As BA mentioned, a cheap low end board like this will ride and is an okay option for someone who is`nt likely to progress much in the first couple of years of riding or they are a kid who is likely to be careless and prone to damaging their equipment. Brands like 5150, Lamar, LTD and some others that you will often find in big box sport stores like Big 5 are all in the same category; they are cheaply mass produced using very base materials that over a short time will loose their structural integrity and wear out.
A decent, intermediate all mountain board from a company such as Atomic, Rome, Ride, Burton, Nitro, Arbor etc can be had for under $300 depending on where you find them and when. Honestly, unless you suspect a loss of interest, you are better off spending a bit more and getting something she can truly grow with.
As for size, a 138 is waaaaaay too small for a girl of 95 pounds. I would be looking at something in the mid to upper 140`s with a soft flex to it. The longer board will be much, much safer and stable at speed. A shorty makes turning easy, but they chatter terribly once the rider learns to really ride and they pick their speed up a bit. A very good board, and one I recommend a lot for female riders just getting into it or at an intermediate stage is the Atomic Fallen Angle. Atomic has several other boards that are less expensive, softer flex and offer shorter sizes. Any Atomic board, even their bottom end will be light years ahead of something such as a 5150. I also am quite fond of Rome boards and they have some good choices that should fit your needs and wallet.
By the way, you`re on the right track about boots being the single most important piece of equipment. Bindings are not nearly so touchy so long as they are reasonable quality. In order of importance I rate #1 boots, #2 board, and #3 bindings.
Go check out Atomic and Rome: I have ridden and demo`d their boards and have found them to be of superior quality, value and performance. There are plenty of great manufacturers out there so check out them too; I just happen to be familiar with these two companies.
Fallen Angle
Tika
Tuesday
ATOMIC SNOWBOARDING start
For Rome, I would suggest you look at the following boards:
Jett
Lo Fi
Blue
Vinyl
Rome Boards | Rome Snowboard Design Syndicate 2009