I think the preds are quite significantly better.
Incendiary rounds aren’t as great as they initially appear. You go from a 1/3 chance of killing an ork in cover with a normal round (or heavy bolter hit) to a ½ with incendiary. That’s better, but it’s not quite an issue of going from useless to amazing. It isn’t enough to make me feel that I must have something able to do that. If the target unit has armour on you are probably going to fire a normal round anyway. Incendiary rounds do work against stuff with crazy cover saves, particularly eldar with their low toughness and armour, but even then these units tend to be small and often spread around over different levels of a ruin where they are very hard to hit many times. Lots of games will be against MEQs that don’t use cover saves against you anyway.
The most obvious advantage the WW has is being able to fire from out of sight. That’s great, if you have something to hide behind, don’t mind inaccuracy, don’t need to move and shoot and so on. The pred has higher front armour and can fire a gun after moving, which the whirwind can only do directly. Bearing in mind that you are going to be coming on from reserve in at least one game in three, all dawn of war games and perhaps some others if you want, that’s important. Knocking out a transport in the early stages of a game like that can make all the difference.
The pred averages 5-6 hits a turn, which is pretty good. A very good ordnance hit can get a lot more than that. It’s difficult to compare the average number of hits over a game to be honest but I think the pred probably gets more. It doesn’t care if units are spread out or small. If you need to blow up a truck or some deff koptas, the pred is way superior. The pred has an autocannon too, so it’s much more dangerous for vehicles. It hits things harder and more often.
On the whole I think that the predator is a fantastic buy. The whirlwind is also good, but is rather more reliant on the right situations coming along. The pred can quite happily fire at just about anything.