I'll just add my thoughts. It is true that Ancestry has the largest database, I think followed by (in order) 23andme, Family Tree DNA and My Heritage. But you also have to consider where the people testing live and where your ancestors lived.
When I first tested mid 2015, Ancestry was only just beginning to test in Australia where I live, and so estimates were that although their database was bigger, FTDNA had more Australian testers, and probably more UK testers where my family came from.
And this proved to be the case for me. I later also tested with Ancestry, and while it is hard to compare number of matches because they use different criteria, I currently have 104 4th cousin or closer with Ancestry, and 210 4th cousins or closer with FTDNA. If I compare the number of cM in common, those 4th cousin or better in Ancestry have 20 cM minimum in common, whereas the FTDNA are 24 cM minimum.
So it seems that with FTDNA I have more and better matches even though their database is smaller overall. And more matches means more possibilities. But for someone in the US, Ancestry will probably give more matches.
A couple of other thoughts. FTDNA has some good analysis tools if you are inclined to do that analysis, whereas Ancestry has more and larger family trees to compare to (though many new testers don't have trees yet). Finally, if you ever wanted to test Y-DNA or mtDNA, FTDNA is the place to go and the same sample can do all three.
All in all, I am very glad I tested first with FTDNA as I got some very good matches there, but it turned out that my most crucial match was on Ancestry. So you have choices to make! I wish you well.