I magnetized mine so I don't have to decide.
But I agree, especially if it's a Wyvren you'll probably want the flamer since it's got that ability to just hide behind a hill and lob shells the flamers will make a better auxiliary weapon vs. out-flankers and deep strikers trying to take it out.
For the Hydra, since it's a little more... single purposed, I tend to take the bolter instead (as well as an HK missile and stubber if I've got the points) to give it a little extra something to do, especially during the typically flyerless turn 1.