In some parts of the country, the EMT-I is basically a paramedic with a small fraction of the education and training. By going from EMT-B to paramedic, you're getting all of the education that you need to go along with those interventions and diagnostics. When you go from EMT-B to EMT-I, you're doing most of what a medic can do, but you're much less prepared to do do the job.
Which choice makes better sense, get some training and do ALS, or get all of the education before doing ALS? EMT-B to paramedic isn't a bigger jump than doing EMT-I, rather you're ensuring that you're fully prepared before practicing ALS. If anything, doing EMT-I rather than doing the paramedic program is a bigger jump, since you're doing the job with much less knowledge to work off of.
EMT-B for a year tops, then medic school, IMO.
Edit: If your employer will treat en EMT-I as a basic, the EMT-I course is an even bigger waste of time. Just do the paramedic program once, rather than do EMT-I, and then have to repeat everything you've learned in I-school.