stand over + seat tube measurements, though most shops use them as a fitting start point really aren't the best way to get your ideal bike size too. I'm leggy enough to be riding a 48cm (I have a Ruby btw), but my upper body and arms are short, so even with my 44cm I had to shorten the stem! If I went solely by inseam I'd be riding a bike that is way too big. If the 51 feels better it likely is. If you can get them to do it - if anyone in the shop really knows how to fit a bike!, have someone who will look at more than just whether or not you can stand over the bike put you on both of them and see which suits your entire body better. They should check your knee/crank position and your reach at very least.