I think that the difference is that a psychiatrist has more time to listen to your problems than a doctor so has a better chance of prescibing the the coreect medication for you.
Unfortunately it seems to be the case that its all a bit hit and miss anyway!
If you look back through the posts, most people will have tried more than one type of drug to find one which works for you.
Also they all seem to come with unpleasent side effects to varying degrees.
I had a very bad reaction to a couple and was/am hesitant to now change even though both my doctor and psychiatrist have advised me to change.
The thought of coming off my meds, the withdrawel is terrible, and the thought that the new meds may make me worse before they make me better is quite daunting.
All I know is, i cant seem to function without the medication and you have to stick with it for a few weeks before the side effects stop.
I wish you luck, dont rule out any more meds they may just help you get your life back.
Keep us posted.