I think you need to ask yourself what you want the board to do. What fins did you use before, how big are you? Are you going to use these fins as a quad? To little info to give any kind of answer.

I personally like larger fins. I like the feeling of resistance/drive of a bigger fin. I don't like "jerky" turns - I much prefer to carve a round(ish) turn with a bit of arc. I don't mean elongated arcs - just the natural arc of the fins/rail line for the pressure/effort I'm applying.

I am trialling a set of AM2 Techflex Thrusters in my 6'1 Hellrazor and so far i dont mind them. I do find that having such a large find i have to get my feet in the sweet spot as they are not as fogiving for someone with lighter build. I do find the feeling of the drive on my bottom turn really nice, i have found that they make my turns a lot more positive as opposed to jerky and when i get my feet right i love the way the board comes around of the top.

So far i'm loving the feel of them..........in a wider tail board like the Hellfire that extra fin area is a good thing especially when it is a little bigger.

I am hanging out to get the Clay Marzo CM2 Techflex fins which hopefully will arrive in the next week and give them whirl.

in the 510hf i'm not liking the am2 t/flex that much..a bit restrictive and holding back what the board is made for...maybe the honeycombs medium flex would allow quicker reaction time??...BUT, the sa3's are a perfect fit...if they felt a little smaller in your bigger version, i would up it to the pancho's...Nev himself likes these options in the 510...so, have a think about those fins...their templates fit better with what the HF is trying to do...in my opinion....