My two cents...I have BBSmart, wife has Empower Pro, used both on my Pearl.
The three areas I'll comment on are:
Speed: Empower Pro - Has the edge...loads and allows you to begin reading quicker.
Email formatting: BBSmart - Formatting of emails is better, though Empower Pro does look good as well. (Subjective, I know, but my wife agrees w/me on this, so I know it's true.
Graphics handling: Empower Pro - Allows loading graphics in-line while you are reading the email...you don't have to wait for them to load in groups as w/BBSmart
There are other difference in features & functions, but in daily use the areas above are the ones that have the biggest impact on me (and I suspect on most folks).
If I was going to purchase again (I bought BBSmart a few weeks ago before the new Empower Pro was available) I'd likely go w/Empower. I don't like how it formats the emails as much as BBSmart, but the speed and in-line graphics loading make Empower an overall superior experience to me. My wife bought Empower Pro last night for her Pearl, and is very happy with it.
The price of both can seem a bit steep, but if you're getting HTML email on your BB (and who isn't these days?!) they make reading those emails a completely different and much improved experience.