Of the two, I'd pick SM64, Reason being that I feel it's a more balanced, and natural sounding package. The Heir 4.a has it's merits and I would consider it an equal, but to simplify a long paragraph's worth of explanation, I'd pick the SM64 for what I listen to. The Earsonics family of IEM's has long been my favorite brand for a long time and I've tried everything to try to trump them for not just value, but outright performance. I think Earsonics is also stocked by some of the best vendors head-fi has as well. Sound Earphones is my particularly favorite as I've spent quite a bit there and had no hangups and they're great to talk to on the phone if you need buying advice. I've skipped posting threads on production IEM's for a long time since I discovered them.
Thank you for your endorsement Gryphus. I appreciate the support and I look forward to our business.