The differences between the two are as vast as the differences between "personality" and "ability."
Hopeless Romantic is a personality aspect. When a sim has this, he/she generates wishes for romantic interactions more frequently than sims do who don't have it. When it comes time to pick their LTW, it will have an influence over the 5 wishes that are automatically generated. Sims with this trait will also be more likely to initiate a romantic interaction autonomously. Basically, it increases their desire for romance, while having absolutely no influence over their success or failure in initiating such interactions (although I believe it makes them more receptive to them).
Master of Seduction is an ability, and has the exact opposite effect. It increases the chances that romantic interactions that they initiate will be successful, but does not increase the frequency that they generate wishes for romantic contact. Likewise, it would have no influence over the 5 generated LTW's they'd be able to choose from upon reaching Young Adult and they would be no more likely to initiate a romantic interaction than any other sim.
Having both would not be a waste at all: you would have a sim who has a higher need for romantic interactions, and also had the ability to get that contact from virtually anyone.