As far as dictionary words (Across all TLDs), properly spelled, NameBio shows the following:
Ending in TY = 923 entries with sales ranging from $100 to $800,000.
Ending in TTY = 29 entries with sales ranging from $100 to $21,000.
While that data alone isn't enough to paint the full picture, it gives an idea about which ending (TY or TTY) might be the most popular go-to.