I have tried both ways to sell. When I first started selling you only had the choice of auction. At that time I felt the auction choice worked really well. On almost every item I had more than one person bidding and I usually received more at the end than I would have priced it for.
Today you can do both and I think it has changed the way people buy on eBay. I now stick to fixed price. I also always add the letters OBO at the end so people know they can contact me and negotiate a lower price for the item. "OBO" stands for Or Best Offer. Putting OBO at then end of my price is not a proven theory but I have had sold a few items this way.
What I do know that has been proven is people rarely go past the third page of listings and often not even the second page. For me this means if there are a lot of similar items being sold I don't post for 30 days because it puts me at the end of the line and my item will not be seen for a long time. I usually choose to sell for 7 days so it is immediately seen.
For items that are rare or there are few listings, I agree that having it up for 30 days gives a wider audience a chance to buy. Basically I look at what items are selling in my category and how many very close items are selling and make a decision on how many days to post for after that.
I'd be interested in any ideas about how many days to sell.