Personally I find both the choices to be valid a lot of the time. Obviously they can be used by jerks to shut down people who don't have the skills to write a patch or plugin, but at least between two developers (i.e. people who have the knowledge to assess ideas from a technical as well as ideal perspective) "patches welcome" and "there's a plugin for that" can be a natural way to continue a conversation, steering it in an important direction.

In both cases the speaker is saying "your idea is relevant, and can be easily taxonomized into a development category". In one case the category is "interesting but not something the core team is going to tackle without someone championing it on trac" (is that you?), in the other the category is "useful idea but so far we are treating it as extension rather than core material" (couldn't a plugin solve your problem?).

IMHO you can always debate the plugin one because if the plugin is popular then maybe it deserves to be in core. If someone says "patches welcome" then the conversation can move into what would be involved in getting a patch rolling and what considerations would have to be borne in mind while working on a patch.

On the other hand, here is the conversation killer that drives me nuts. It's not a full expression, but merely a keyword that can, in 1 second, kill a conversation about a development idea: Bikeshed. The accusation is that your idea is trivial and a matter of taste, and that discussing it is a waste of time. It's a useful concept when a topic has been debated to death already (WP-Hackers being the bikeshed-debate-HQ for WordPress), but in personal conversations it can be really insulting, especially when you are pretty sure that your idea hasn't been talked to death yet. Once 'bikeshed' has been whipped out you are not only faced with the challenge of explaining your idea, you have to first defend the entire topic against the accusation of being a waste of time. Depressing.

Great poll by the way! I hope everyone can take to heart that both options are not necessarily attacks, but can be a healthy part of your morning developer discussion ;)