Those pics look like they are the GM size. I'm also 5'1" and I usually carry the PM. Since the roses NF was only available in MM, I compromised. By keeping the sides cinched in, the MM looks fine. The GM graffiti was unworkable for me.
The size of the bag will also be determined by what you carry. The Neverfull PM is a pretty small bag and would fit the contents of a Speedy 25. The Neverfull MM is much larger but can be made to look smaller by pulling in the drawstrings of the purse.
I'm 5'3" and have an MM and GM. I use to think the MM was perfect for me, but now I love the GM size and the MM is sort of small. I also had a PM (for about a minute) and sold it immediately because it was just too small for my needs.
I would recommend the Neverfull MM - you can make it small or big depending on your needs.
I have also been reviewing NF threads especially the clubhouse. I just ordered the Damier NF MM. I'm 5'4 but really thought about getting the GM since I will be using it as a diaper bag as well. However, my son won't always be in diapers and I want to be able to carry it after that without thinking it's too big......
I have the neverfull PM and love the size ... peace43 is correct that, contents wise, it fits the same as a Speedy 25. I am contemplating a damier neverfull MM for a bit more room ... to use as a tote, or for days when I have a bit more to fit in.
I'm 5'2 and have an MM. I think it's the perfect size! You can fit so much stuff into it. I use it as a purse/diaper bag combo. I can fit all of my stuff in it plus diapers, wipes, changing pad and snack for my daughter. I can even get a few books for her in it as well.