With all things being equal the classic differences between the fl and rl cabs are the fl cab is going to be tighter in the bass and have a more focused sound. While the rl cab will be a little looser in the bass, not so laserbeam-ish, and some would say it covers more musical styles.
Put it this way, if somebody came up to me and said they only play stuff like Meshuggah or Machine Head I'd say check out the fl cab first. Whereas if someone said they play Meshuggah, but also ventures into STP or Black Crowes territory I'd be inclined to suggest looking into rl cabs first. It is by no means a hard fast rule since there are plenty of people who prefer fl cabs that don't play metal and metal players who prefer a rl cab, it's just where I would suggest beginning the search for a cab, or in your case, if you can't play on them for yourself, to at least keep in mind.
I hope that helps a little more.
And my reasons are those aforementioned. I wanna unleash the brutality in Herbert, get tighter sound with faster attack. Übercab is a great cab but ´cause I´m playing metal I´m gonna use Front loaded D cab. To me G12K-100 is very balanced sounding element that´s great for those heavy, metal tones. Delivers them with authority. If I would still use the Übercab I would change G12T-75s to Eminence Wizards. To me that´s even better combination than original.