Neither is better it's just a matter of personal preference. However, some background and rules of thumb so that folks can have some knowledge.
Typically single trailer fins are placed further back on the tail. This will widen the arc of a turn and tends to "lock" the tail down when you try and slide the tail out - all things being equal. That is to say if you have a 4 inch trailer fin it will track very straight and it will be hard to break the tail loose. Typically if you look at skim style boards with a single trailing fin those boards have a fairly shallow fin - less than 2 inches.
Twin fins typically have the fins closer to the rails examples of this configuration include most of the Inland Surfers and most of the TWP Comp X line. These fins are also pushed further forward of the tail, in comparison to the single trailer fins. The further forward the fins, the loose the tail will be. When you go to slide the tail out, the board will pivot around the fins. It's easy to see that moving the pivot point forward would give a feeling of looseness in the tail.
Now we come down to what is referred to as drive. Drive is that feeling that the board is moving forward. It's not quite acceleration, perse. Squirt is that sense of rapid acceleration, but drive is how fast the board will go in comparison to the speed of the wake itself.
Drive is greatly affected by fin area. The greater the fin area, the more drive that is experienced. Just by volume alone twin fins will offer more drive than a single fin...IF the fins in question are of relatively the same size. This can be a decided advantage say in situations where you try a surface three and don't quite make it - you're about to fade out the back, the twin fin will give you more drive to catch back up with the wake.
If you are more of a surf style rider - huge slayshes or you like carving and always riding the board with the tail in the back, then the twin fin is better suited for your style of riding than the single fin.
The single fin offers some decided advantages when your style is more skim oriented. The basis of many skim style tricks is the shuv. Briefly, the rider pulls his feet above the board and causes it to rotate 180 degrees beneath him. When he lands back down, the board is revert - meaning that the tail is now where the nose used to be. At this stage, the trailing fin is now at the nose, the shallower single fin is a decided advantage here. Rail mounted fins that were 4 inches deep would have a tendency to catch in the face of the wake and would also require that the board be ollied HIGHER in order to do the shuv - about 3 inches higher in order for the fins to clear on the rotation.
There are some other issues that need to be considered also. Two very important concepts are toe-in and cant. These refer to the way the fins are angled. Toe is just like the toe-in on the front wheels of your car - it's how angled in the leading edges of the fin are, in comparison to the trailing edge. Cant is the angle of the fins from the base towards the tip OUT from the center towards the rail.
These two concepts provide lift, drive, aid turning and squirt. If you look at most boards offered by large manufacturers - say a hyperlite or a cwb, the fins are straight up and down (no cant) and straight ahead (no toe), when combined with the relatively shallow fins these boards are typically slow as molasses running uphill in February
and are unresponsive. These manufacturers do this because it's easier and cheaper than attempting to build in toe and cant.
Probably more info than you wanted - but now you have some basis for assessing the quality of the ride of a board you are considering.
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