I work for my hometown volunteer EMS and a separate private service that is out of town.
The private service gets most of its mileage from the interfacility (boring) transports, but does cover it's hometown's emergency needs, as well as some outlying areas served by First Responder units.
Yes, 911 EMS action is usually more exciting. But, if you want to learn more about medicine, interfacility transport can also be very interesting, because you'll run into things that you might never see in EMS. Also, I've found that these types of transports are great ways to work on your patient contact skills. Occasionally transports will be a bit more exciting, for example when a helicopter isn't available for an emergency transport, and you end up taking a patient from one hospital to another one 50 miles away with your lights and siren on. But that's usually just more exciting for the driver, and even then it gets old after about 10 minutes.
If you're paid by the assignment, a private service might be better for pay...personally I get $20 per patient, plus 50 cents a mile for the transport. The same goes for any emergency calls I respond to...these usually take about 60 minutes at most, so you're looking at something that boils down to about 20 bucks an hour...for a transport that takes 4 hours, you might make more money, but from a per hour perspective it isn't so great...you might be out of service for 4 hours but only make 50 bucks.
My vote is for a busy 911 service if you like adrenaline and trauma
