That is a really difficult question to answer because the answers is simply "it depends." It will vary greatly from company to company, department to department, manager to manager, and even down to the colleague you're sitting next to.
As somebody who has worked as a consultant on both salary and hourly basis, I can be the first to tell you it makes no difference between whether I worked salary or hourly, it all came down to the project manager and the company culture.
You will feel more pressure if you have a crappy project manager (PM), you will feel less pressure if you're working with a PM who knows his/her stuff. Any project manager worth his/her salt will know how to properly estimate development of any given feature and consequences of such features based on their development methodology, historical develvery experience, and team capabilities. Most people forget to allocate enough testing time, and if they do, they don't allocate enough time to fix the problems found during testing. Thus, finding project managers who are aware of these problems are far and few inbetween unfortunatley.
If a reputable company hires you hourly, chances are you're going to end up with a good project manager since they probably have already (accurately) estimated how much time a project will take to complete, and did a cost-benefit analysis and decided to pick you up hourly. In most cases, it means less developmental hurdles than salary actually. However, if a reputable company hires you hourly, and you're finding that you're working overtime constantly, consider looking inwards for a problem.