They are both VERY different, almost impossible to compare them.
Assassin to Sinx is, sure, a grindfest of wonderful grimtooth spam, but upon getting to the wonder of sinx, you have several different choices for builds. My favorite for PvM is Soul Breaker (also known as Soul Destroyer), especially when you get yourself a set of elemental haes and an ice pick, as well as a quad zipperbear orc axe (best....weapon...everrrrrr). Pump str/int, get about 50 dex give or take and pump the rest into whatever you want (...speed is sort of useless, as well as luk, so I always pump vit or get outrageous dex...fast cast is win).
Also with Sinx you have the wonderful choice of enchant deadly poison, which is an essay on its own...oh the places you can go with that. And the leeching you can do~
Stalker though...leveling wise, and mostly in PvM you will most likely play as a bow stalker. In my opinion, if you are going to do that just make a Sniper. Stalkers are only good for PvP anyway. Anything you do with a stalker can be done better with other classes, except for their stripping, which is sort of useless in PvM