Yes, there have been some high-profile assaults, but naturally the vast, vast majority of people who use MAX are perfectly fine. If it makes you feel better, a suggestion is to sit as near to the front of the train as possible so as to be directly behind the "driver".

As for a cab ride to PDX, the last I did it was in 2006 and it was $27, and that was to the south waterfront area, which would be a slightly longer ride than downtown. Taking a cab would also eliminate the hassle of waiting for the hotel shuttle once you got back to PDX, since walking from the terminal to the hotel would not be a very safe thing to do at night (not pedestrian friendly).

Since two MAX tickets would be about $4, the net cost of the cab ride would be around $25 including tip. It's just a matter of cost versus convenience.