I tried both 48 and 51. Both of them were comfortable but I had to raise the seatpost really high which didn't look so nice and 51 was a little bit more upright. Also I liked the color scheme of Se Ultegra better. At the end I bought the 51 Synapse Se Ultegra Disc. Awesome bike. Financially I will be broke for a long time but still it worth it...I guess. Time will tell.