I used to think Speedys where too common & horrendous too - until I saw it on TPF with a scarf, just like you =) I also had the same size question as you when I was choosing mine. I went to the store and immediately thought the 25 was way too small for my needs (and height), so I chose the 30 in Damier Canvas so I wouldn't have to worry about the vachetta. Just so you know, I'm 5'6" and 115 lbs - only 2" shorter than you ^_^
If I were you, I would go with the 30, because the 25 is pretty small/hand held. Of course, I'm probably a bit biased, because I have one, but I love the size and how much I can put in it depending on where I'm going - friends say I pull amazing things out of my bag =D If you want a small bag, go with the 25, but for an everyday multi-purpose bag, go with the 30. It def. goes from day-to-night easily.
Also, I haven't seen many 25s accessorized with scarves, but when I think the 30 is perfect for attaching a bandeau to. At first, I thought it was a little too big, but I got used to the look of my Damier 30 + Sweet Flowers bandeau in red, and now I love it.
Hope that helped!
P.S. If you hate the sag of Speedys like I do, buy a Speedy Shaper on eBay - I love my Speedy even more now that I have a shaper/organizer in it!