Monitors are often more of a pain to set up live, plus amps are built to handle the wear and tear of gigging while monitors are not. With monitors, you want to have the two positioned so that they form an equilateral triangle with your head, which probably means getting stands for them. With an amp, position isn't as important, plus if your amp is large enough, it can just sit on the floor, maybe leaning on something so it's tilted up at an angle, and you'll be able to hear it. Small monitors on the floor won't work so well in that regard.
Also, monitors are stereo while most amps are mono, which may make a difference because most of the time, you'd be playing through a mono connection to the house mixer. With a mono amp, you won't be hearing things that everyone else isn't hearing, which may or may not be important to you.
Studio monitors for $150 aren't going to be any good. Just because it's labelled a monitor (as opposed to consumer-grade speakers, where the sound is intentionally not flat, so as to provide a "better" listening experience for the majority of people) doesn't mean it will be accurate. $150 won't get you a pair of monitors that's anywhere near large, so the bass frequencies will be skewed, since the speaker cone won't be big enough to properly represent them.