I was trying to answer this question using a couple of web-tools.
Firstly, I looked at relative research interest to particular topic using GoPubMed tools. This result is based on number of publications per year, indexed in PubMed.

I decide to compare: Gene Therapy:

Stem Cell:

RNAi:

microRNA:

Next, I analyzed public interest, using Google Trends

Google Trends analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. We then show you a graph with the results — our search-volume graph — plotted on a linear scale.

1. Stem Cell (red) vs Gene Therapy (blue)

2. Embryonic Stem Cell (blue) vs Adult Stem Cell (red)

Finally, fun stuff - Google Fight!