In my very limited experience... maternity jeans suck. Let me explain why. You do have several options to chose from. Here's a photo with a great explanation of the different styles available from Old Navy Maternity's website.
I purchased the pair on the far left with my mom over Thanksgiving; my first maternity shopping experience. I was only about 11 weeks at the time, and thrilled that these jeans fit SO well and only set me back $20. That was before the expansion began.
Since I've always had such a little booty I honestly didn't think anything would change in that department (This is when women who have had children before laugh at my ignorance). So now at 16 weeks and 3 days I've rounded out quite nicely-- and evenly. At least I won't tip over ;) These jeans still fit now... but they slide right off my rounded out figure! SO uncomfortable.

Next stop: Craigslist, for some secret fit belly designer jeans originally purchased from A Pea In the Pod. I'm pretty proud of this purchase. I love me some designer jeans, but at $190 a pop for a pair of jeans I'll wear for a year out of my life... no thank you. The large panel looks gross I know, but actually that part of it is really comfortable. It feels like you're wearing one of the camisoles that hold everything together, and you don't have to worry about the pants sliding down your rounded body. The part that doesn't feel good is where the jeans ended and the stretchy panel began. It hit me right above my butt crack, and right on my hip bones so it felt like I was rolling gangster style, even though it didn't look that way. Bummer!
So the verdict? I'm sticking to my Old Navy leggings (I found them on sale for $10) with tunic tops or sweater dresses and boots whenever possible. At this point my maternity jeans are LESS comfortable than my pre-pregnancy jeans with the hair-tie-through-the-button-hole trick. My advice to you would be to save your cash for other maternity items you can't live without. Thankfully I'm out just $60, which I plan to get back via craigslist after I've gotten good use of everything :)