I think you should go for it ASAP if you can get in the class.
I started as a First Responder on a EMS explorer post, then I went to EMT-B then I went to EMT - Intermediate and I am a Paramedic student. I can tell you this, if you plan on going on or even if your not sure and thinking of going to Paramedic later and you feel you need more experience, education, and that extra edge then I Recommend intermediate.
It will help you get accepted over EMT-B's in some paramedic programs
You will learn your basic cardiac drugs and rhythms, and start to calculate basic drug calculations such as Lido. you'll get plenty of I.V. experience, and in the mean time no Transporting 911 agency who runs 1 emt and 1 medic on a rig or who runs ILS rigs wants to hire a EMT-B that can do nothing, when they can hire a EMT-I pay him or her almost the same rate and get everything out of them.
If you do not plan on getting your paramedic and you want to do EMT as a part time or volunteer thing for your EMS career I also highly recommend Intermediate.
Now here are some reasons I don't recommend Intermediate
If you live in a Major metropolitan city with about 1 million or so people in or around it there's no need for Intermediates, you may not even be able to use any of your intermediate skills, some states or county's revert you back to a basic, and worse of all, if you ever plan on moving no other state will accept your intermediate rating unless you get Nationally Registry Intermediate, and even then only a handful of states that I know of widely accept it. Intermediate is extremely fast pace for what your excepted to learn, know and perform, you can now push narcotics, use advance airways, and making a c couple clinical decision making skills regarding drugs that if you screw up, could end someone's life....
So as an Intermediate myself that's what I have to say take it for what's it worth, and also please keep in mind I live in Portland Oregon, the info I gave you could be completely irrelevant to your state and or city.
ALso if you plan on doing EMS as a fulltime career to support yourself and live of off alone (haha as if you could j/k) You must get your paramedic, atleast if your on in Oregon or Wahsington.....
if you plan on doing fire as a career then Intermediate might serve you well if you have other things to bring to the table and dont want all the extra education in EMS.