Jassen,
Wlecome to Jetcareers.com! I did the 90 day Airline Career Pilot Porgram at ATP not too long ago at the Sacramento location. I would be more than happy to answer some of your questions...
First off, I have never heard of Monarch Air, so I can't compare them to ATP. If you decide to do the self-paced program in Portland, which is actually Salem, OR, you will be very happy. It is a nice airport, and the instructor out there, Jesse, is great! I would also recommend looking at Sacramento

Anyway, ATP is a good choice, if you have any more questions post them here.