The model cement is actually the thin watery cement in a thick filler. Both actually glue by chemically melting the plastic and bonding it like a weld. There are two advantages of using the thick stuff, one is if you have uneven surfaces the glue tends to fill in the gaps. Secondly, it doesn't dry quite a quickly so you have a bit more time to adjust the parts. The bad part of that is that it globs and can leave a messy joint.
The others are right, I think using the thin cement results in a much better looking job, the problem is that if I don't use it in a very (emphasises on "very") well ventilated room with exhaust fans, I get a reaction to the fumes very quickly. :curse: That's the only down side I see.