I would schedule a 25 man for when the majority of people will show up and then pug the remaining spots. This will have some benefits:
* The other guild members will feel left out and get their arses in gear to attend the next 25.
* Pugging might give you raiders that are able to raid those times. And a pug in a raid is the perfect interview..
Keep the 25 man run the same time every week and soon people will find ways to plan their time to attend. Nothing motivates raiders more than seeing other guildies getting better loot from a raid that they could've attended... but didn't.