I would stress highly that $$$ is not the most important thing. You have to have some enjoyment & passion for the field you enter. If you love your job, everything else usually falls into place nicely.
Psychiatry IMHO pays more money. I'm basing this opinion on a few websites I've seen that have presented the average salary for the individual. There are more areas of the country that are in need of psychiatrists vs FM or IM. The lifestyle is not as intense (at least by my opinion) in terms of workload-you're usually not awoken at 4am to a code blue where your patient is about to die, though that really depends on the individual. I've known several IM doctors who mentioned they just could never do psychiatry.
Despite what I mentioned, I am in no way implying that IM or FM are inferior to psychiatry in terms of importance. The above are based on simple market factors (which could change in the future--though it would take several years for that change to happen), and my own opinions. I find psychiatry easy because I love it, but I also like the part of it where I'm not being woken up in the middle of the night several times a night, several times a week, several times a month.
I would also stress to not go into psychiatry simply because you may find it easier. Like I said I find it easier because I love the field. The last thing I'd want is someone who wants an easy job & not pull in the hard work to enter this field. Psychiatry, like any field will have those times where you got to put in the extra work to give your patient the right care.