MPs have a fixed rate and a fixed time. So you pay $40 and you get 1 hour. If you can't get your nut in an hour then you either pay for more time or walk out with blue balls.
DR girls will cost more but time is more flexible. Now, you may get a clock watcher who will give you only an hour that you just paid $100 for. Or you may get a girl that will hang out with you and you get multiple pops.
MPs mean you do it on site. So it's their rooms and beds. Sometimes the beds aren't so great or non-existent.
DR girl you usually take back to your room or to a short time hotel.
Reliability for either is how much personality and effort you put into it. If you get a good vibe when chatting with her, then you will likely have a good session. If don't get a good vibe, your best bet is to walk. There are always others. This doesn't mean that she can't talk a good game and then get into the room and lay there like a dead fish. It happens to everyone at least once. Just go NEXT and move on.
Really, it comes down to a matter of preference. Some guys find the MPs boring and don't like them. Other's will only do the MPs because they prefer the quick in and out. I enjoy both since I rather not limit myself.
You are likely not going to leave the DR once you walk in other than to take the chica to where you are staying if it's not there. If you do manage to leave it will be to ZB or NF which is where most new guys wind up. Nothing wrong with doing it that way either. You always can plan another to explore what you really like.
Cheers!