Both are really good.
Which is better, in duels depends mainly on what you are fighting against. For team pvp I'd deffo recommend /ff, but both are really good for duels
If that can help, I can use my fire/ff vs some toon of you sometime, so you can check what it's good and weak at
After the many butt-whoopings Blasphemy's givin' me, i'd say go with the Fire/FF. 
Plus it isn't a common build to run across, in zone's or Arena, so that in itself has its advantages.
@SinghMC
Lol, errm let me make clear that by 'butt-whoopings' he is not meaning anything +18. And yes I suppose that not so many people is used to fighting vs /ff as vs /rad, which is an advantage. But fire/rad is very good too, mainly for duels.
Thanx guyz, i have been testing my Fire/Force Feild in BB & i have to say my defence is awsome & im loving it .
Hi guyz, i have 2 Cont's just testing them Atm, 1 Fire/Force Feild & 1 Fire/Rad.
I have only really played the Fire/Force Feild, But before i continue, would the Fire/Rad be a better choise??
Because i am mainly thinking PVP, with Fire/Force Feild, it is ok @ PvE (not tested PvP yet). But because i am mainly going to be a pvper with this alt, Should have i picked Rad for the Debuffs Ect?
I was wondering beccause i know Force Feild is good @ PvP @ higher lvl, Sirens for example, but @ low lvl's Force Feild is mostly a PvE Build (adleast untill i lvl higher & respec).
TY for your replies coz i dont know what to do & i know you guyz know your stuff