I don't have an iStick, but they get great reviews. I would definitely get the 50 watt. You might find she wants more power when she gets used to it, or if her atty choices change. Plus, more battery life, and she won't be pushing the device as hard.
For instance, I got my iPV3 for the reason of dual batteries, I don't need the full power, or even two thirds of it, but I have found myself vaping around 55 watts as time has passed. If I had bought a 50 watt box, I'd be desiring more power already. The thought of 55 watts when I got it was something I never thought I'd use. That changed quickly. Since the cost isn't that different, I say 50 watt.
And if batteries and a charger aren't an issue, look at the Sigelei 50 watt plus, or the iPV Mini 2. I prefer to have a user replaceable battery, but I understand not everyone feels that way. Both of those can be had for $50 or so.