Shai
This maybe the frequently asked question in pvp history. I don't have the right to say what's good or not good, but i'll say this on my perspective. If you're a non-donor, RP-wise, i suggest FM. Because even if you don't have donor items you can get high asal while if Harle, I think you will get around 300m hop. Max HOP for a donor harle is like 690m if i'm not mistaken. And I think that 300m non-donor hop won't do any good at siege cos players there have STA gears.
Strategy-wise, Harle can always DI and HOP and hope that your target is on STR gear so you can still one hit them. While with a non-donor FM, I think you can still one hit players even in their STA gears. Well, someone correct me if i'm wrong. Nevertheless, Harle if you're a Donor. And FM if you're non-donor.