I a word - YES - it is more stressful than applying for grad school. I would not apply for internship unless you are really committed to going. It's too much da%^n work.
I did 5 years for the same reasons you mentioned - it gave me an opportunity to publish more, collect ancillary data to take with me, and finish and defend my dissertation prior to leaving for internship. I had proposed the spring of my 3rd year, but there is no way I would have had all of the data collected and the manuscript written by the end of my 4th year. Going the 5 year route, I had plenty of time. Having seen my peers try to finish their dissertations during internship, all I can say is that I am really happy that I took the 5 years. Having my dissertation done also allowed me to jump right into manuscript writing with my new mentor on internship.
Also, I did my internship at one of the most competitive "reasearch-oriented" internships, and I wouldn't discount the importance of having meaningful and diverse clinical experiences on your vita. They're not necessarily looking for tons of hours, but they recognize that this is a clinical year and that interns are going to be working in clinical settings for most of the time - as a result, they ARE looking for good researchers and good clinicians. Completing an externship - especially if you will have exposure to a certain population of interest - can give you a leg up. If you can tie that clinical experience to your research interests, that's even better. For me, taking the 5th year gave me an opportunity to do an externship that I talked about quite a bit in my applications...
Just some things to consider... Good luck!