The most important thing to consider is whether or not she needs a smaller mouthpiece, and then get the smaller size if needed. There is nothing about being a smaller person that really makes a difference on needing a smaller or lighter regulator. (No one out there looking for a bigger heavier regulator for a bigger diver, right?)
Having said that, the Mikron is a nice regulator and a good choice. But a smaller 2nd stage tends to underperform a bit when compared to a larger model. The Mikron Work Of Breathing is 1.1 joules/liter, while the Core is 0.8 joules/liter. Both are pretty good WOB specs and you may never notice the difference, but at least in the lab they are not the same. The Core doesn't weigh much more than the Mikron, and is $40 less. Probably something like half of that weight difference comes from the rubber hose on the Core versus the Miflex hose on the Mikron. (The 1st stages are almost identical.)