I tried both BBsmart and Empower extensively. What I found was that each rendered email differently, usually in subtle ways which depends upon the particular characteristics of email you receive. RIM has this infernal policy of mutilating HTML email on their servers before you can receive it. If you receive HTML mail regularly from a particular source, it seems to arrive mutilated in a consistant fashion. Thus, when it comes to displaying such email, there is a difference between these two email viewers.
I based MY final choice upon the following(YOUR experience will differ
1. For my own needs, both Empower and BBsmart feature sets were satisfactory, so that was a non-issue.
2. I liked the fact that Empower occupies a smaller memory footprint than BBsmart.
3. I liked the fact that Empower was less expensive than BBsmart.
4. For the email which I typically receive, Empower renders it faster than BBsmart.
5. For the email which I typically receive, Empower rendered it in a fashion which I preferred over BBsmart.
So, I kept the Empower email viewer.
I think the only way to successfully choose between email viewers is to take the time to trial all the viewers and select the one which serves your needs best. The good news is that the trial versions are fully featured, so you can get the complete experience with each, before making a final choice.