I'm a huge believe in enclosed trailers because they are more secure and because of the advertising they provide.
Our enclosed trailers are like a giant rolling billboard driving all over town and parked in high end neighborhoods every day.
So I am a big proponent. But you need to be careful. If you think that these big billboards alone are going to drastically increase the # of calls you get, you may be in for a surprise. I would say we get a little higher % of calls now, since we've gone to enclosed trailers with big lettering all over them, but not a huge increase. What they do is increase your company name recognition, which is different.
You need both. Name recognition AND advertising. They work hand and hand. You could have all the advertising in the world but if the person seeing the ad has never heard of your company, it won't be as effective. Conversely, you could have all the name recognition in the world too. But when someone needs a new lawn care company and they go to the internet and type in something like, "lawn care companies in Plano Texas" and your site doesn't come up, well, your name recognition didn't help you at all in that case. They're going to go with one of the companies who's site DID pop up.
Where it helps is when you have BOTH. Say someone opens the yellow pages or does an internet search. And then they see your ad or your website. Maybe they click on your site and see your company logo, a photo of one of your trucks, etc. Now they're thinking, "Oh yah!!! I know these guys. They been doing that house over on the corner for years! I see them all over town. I'm gonna call this company." Instantly, you have credibility in their eyes. And I can't tell you how much that is worth. It changes the whole game!
If you had to make a choice between the two I'd go with advertising over name recognition (that is, advertising first....lettered up trailers second). You'll get more calls by spending your money on advertising. But I'd make it my second priority to eventually get some trucks and trailers that would really create some name recognition. Because that's worth gold.