Well you use both (check Chandy's post). Fthrow is better, Dthrow gives the opponent more options.
Also I think people just need realize you are a freaking Marth clone. Don't just go for fish for down tilts, play like a wiener with your top tier movement and grab range. Like Sheik and Fox players should be annoyed approaching you. Vs Falco, Marth players like to Jab in between the Laser to Aerial hit stun or dash away, dash back + grab right in between it (check PPMD vs Zang for a good example of this). Roy can do the same thing. Forward B even works and can **** the Falco's life since this is a kill move. Even at lower percents, just do > ^ ^ to setup the knockdown for a tech chase.