Buy Fluorescent Bulb With Warranty Or Without In This Case?
If I'm understanding correctly, E14 is the bulb base size (14 millimeter, right?). That means that the bulb is about $10. Which means that the cost in your time and energy to make good on the warranty (not to mention storing and then finding the original receipt again) is probably more than just buying another bulb. 

That's not to say that you shouldn't buy the more expensive bulb, if you think it's better made go for it, but I doubt that you'll ever make good on the warranty.

As to why they burn out: How long did the 15W last? And what is the hood rated for (they usually have a sticker inside somewhere that lists maximum bulb size)? I'd put the 9W in, run it for a few hours and see how hot it seems to be by touching the outside of the hood. If it's really hot, then drill it out for airflow, if it's not, then leave it.