Which should I be using? I've been using BAM for years. Is BAM being phased out? If I use this will all of my stats from BAM be ported over to Science United?

I don't think BAM data can be sync to SU.
SU is design to auto distribution the project, which means the user can only choice project by research type or location which is good for people who don't want registered bunch of project account. (of course you still can add/remove project on BOINC client, but they will not sync to SU server)

no, BAM is not phasing out. What made you think that?
And no, nothing will be "ported" since SU made the BAD decision to use new, anonymous accounts for every project you get added via SU (using a fake email).
Meaning, you will get a new cpid and be a "new user" for all projects. I think there is NO way to synch this.

I tried su for a few days, but i am already getting "cpid error" for some projects. Think i´ll switch back to using bam with most of my systems.