The real difference?
About 35 bucks (the 194 is less expensive)
The usable difference lies in the ratcheting system.
The 194's ratchet is on all the time (unless you turn it off with the slide switch on the side). That means that the center pulley only rotates in one direction (freely when you pull in; acts as a drag when letting out).
The 2629 is a "Ratchamatic" block - the ratchet is pressure sensitive and only engages when there's tension in the line.
I find the 2629 to be easier to deal with in tacks - the line lets out when you need it to, yet still has the grip assistance of the ratchet when tension is on the sheet.
Downside - if you drop the sheet, you're screwed. It'll run out like you-know-what through a goose. Case in point:
The result of dropping the sheet when on the wire. (That is me, BTW - still on the wire and still steering the boat.)
Some people have claimed that the cleat angle on the 2629 is hard to adjust, but I haven't had that problem.
The 194 is basic and bulletproof. I've used them for years.