Well, first, don't let the home care person who brings out your machine try to fit you in a hurry and then leave! After you have preliminary decision, I'd lay down with it on while the RT is still there, and make sure there are no gaps leaking air or pain.... and by this I mean ANY air or ANY discomfort. It should be leak tight and comfortable.
If you keep your mouth closed when you sleep, you probably don't need a mask that covers your nose and mouth both.
I dislike the nasal pillow style (personally) they hurt my nose. I use the nasal mask which is shaped like a triangle and fits over my entire nose. It has a gel insert (IMHO very important for comfort) I think its made by Respironics, but other manufacturers have them too. But, with a nasal mask, reading glasses don't work... some masks are quieter than others, due to better management of the air blowing out the vent. If you sleep in bed with someone else, this is an important consideration too.
To a lesser extent, the type of headgear strap configuration also affects how much you will like it ... comfort... *how bad your "headgear hair" looks in the morning 8)
You could go online to a site such as cpap supply, etc, and look at the various choices. Same masks are used for cpap as bipap.
Don't settle for what you're not sure of. If the mask you opt for does not work right, call the provider back and have the RT come back out again.
Same goes for the machine pressure settings, until you're used to it, you'll probably want a long ramp up time to full pressure. Ask if your machine is set to the max or not for this. Understanding about your machine's humidifier is important too, and what setting you should start out with (that is easy to change even in the dark at night, just a twist of a knob)
When I finally found the right mask, and machine was set correctly, it made such a huge difference, and I don't even consider laying down without it. Even if I get up early to let the cat in and dog out, I put it back on again to snooze a while.
good luck!