i think gwen's is too bright while the other one is good. I think red lips can look very sexy if done right. in a more subtle tone of red than bright wham bam lipstick. and if red lips are worn the rest of the makeup should be neutral colored and light. but they can definitely look very sexy.
Gwen Stefani is a good example. She's an older chick with a great body & cool overall vibe. But without makeup I doubt most people would even recognize her, from the neck up. Which isn't a good thing.
It's a cliche but natural is always better. Always, always, always. Not that there's anything wrong with adding some enhancement (makeup, whatever) but that stuff should complement the natural beauty, not the other way around.
Red lips are amazing. It's a classy look but you can also make it really edgy. I know few people who can't rock red lips. It can look good on anyone as long as it doesn't clash with the rest of your make up.