I need to create a simple UI application but I'm confused which one of these two could be better feature wise. Can somebody suggest a link where I can learn the difference between these two, or anything like that.
TinyX is an X11 server. It understand most of X protocols.
DirectFB is not implementing X11 protocols. It is a library, and it is probably obsolete (not maintained very much by an active community, ...) or phased out.
Did you consider Wayland ? It is supported by GTK3 and by Qt5 !