IUI - which is better Clomid vs Letrozole?

Ladies,

I used 150mg Clomid and ovidrel trigger for my IUI cycle #1 and it failed plus developed a 6cm cyst. My IUY cycle#2 , I took clomid but this was cancelled because my estrogen was soo high plus I ovulated too early. I just had a natural IUI cycle #3 with trigger shot which failed again, my period came very early! My doctor said she will put me on progesterone and is recommending I try Letrozole.

Which was is better btwn the 2? Any insight will be appreciated.