My understanding of this: Even though you can probably TAKE the ACLS course as an EMT-B, you probably won't be able to use the skills on a service anywhere. You're better off with EMT-I if you want more skills that you can actually use.
Depending on thescope of practice in your area the EMT-I may be albe to use all of the ACLS cert. It is, however no the other way around. Certification is the key, a single class changes nothing with regards to scope of practice.
If you're in LA county, forget about it. Scope of practice is so ridiculously small for EMTs (even paramedics) that I sometimes think all I can do is give O2 and transport... ugh