I have used both and prefer the Slimblade although the Expert Mouse has features in its favor as well.
I used the EM for about 2 years but was initially disappointed because of the excessive wrist extension needed to operate it.Soon after, I inset it at an angle into a tray and was very happy with it. When I got anErgoDox I wanted to incorporate the EM into it but ran into some clearance snags which I was able to solve with the Slimblade.
Keep in mind that I haven't used either trackball in an unmodified form enough to give a completely fair analysis but here goes.
I prefer the EM switches. They seem to have a larger "sweet spot" to hit and the Omron switches are much more robust than the Slimblades. While the switchplate surface on the SB is large, unless you hit them close to the 3 o'clock or 9 o'clock area, they aren't as easy to depress.
The EM also has 4 buttons that allow 6 assignable functions which is nice. The SB doesn't have provisions for chording but it offers other controls which I never bothered to set up so I can't comment on that.
In the SB's favor, the scrolling feature is nothing short of great. While the EM scroll ring is nice, it is somewhat scratchy compared to the smooth rotating ball on the SB. Don't worry about the spinning ball affecting the cursor: it doesn't.
The lower profile of the SB is also a big plus. Much less wrist extension.
They are both good trackballs in my opinion with the SB being a little better. If you are considering either, I would recommend trying both.