The amount of time that is required for clinical (in hospital) and the internship (field) varies widely, as does what the student/intern is allowed to do. In some states the amount of time an internship lasts is as little as 200 hours with 40 ALS pt contacts...think about it, that's less than a month of full time work. Do you really think that will help truly prepare someone to work in the field on their own? Of course, that isn't necessarily a problem; some agencies expect this, and are willing to spend the time, effort, and money in getting their new hires up to speed before letting them loose on their own...strange, but it's usually these agencies that don't have a problem with people leaving... Others expect people to come in fully prepared and throw them to the wolves right off the bat...not necessarily a problem if your internship consisted of well over 1000 hours, but if it was short...could be a problem. This is where prior EMT experience could help. (and by EMT experience I mean working in a 911 system paired with a paramedic...though that doesn't always work out)
If you come into the internship already being comfortable in talking with pt's, police, nurses, doc's, family, bystanders, nut jobs, and everyone else we encounter, it makes things much easier; not having to learn how to gain a rapport with people gives you more time to focus on how to be a paramedic. The same goes for the basic skills; backboarding, splinting and the like. While it doesn't take much time to master those, it still takes time away from being able to learn other things, and, depending on where you go, you may not have the luxury of a lot of time.
Unfortunately, most of the jobs that people find as EMT's are not 911 related, or only a tiny percentage of the calls are; interfacility transports all the way. While this can be a learning experience, more than likely it won't be, and will lead to learning poor habits, attitudes and lousy pt care. So really, that isn't recommended.
There really isn't a perfect answer. You need to look at your own comfort level in talking with random people and being able to function in stressful situations, the length of the paramedic internship, how the local services (or wherever you want to work) train their new medics, and the type of job you'd really get as an EMT. For some it may be better to work as an EMT for a year, or during medic school. For others it won't.