My vote is for the Sena case. There are several reasons for this. The main reason is the magnetic closure. It allows one handed use with the case and is just cooler, to me. Also, Sena offers nicer designs than Piel Frama. For example, my x50v sits in a Black/Red case. Almost daily, I get compliments about it. Also, it's great to be able to sync in the cradle without removing the case. According to other aximsite users, the regular Sena case allows you to use it with the 1800 battery from mugen. To them, it fits great.
The Piel Frama cases are good, but the snap closure turns me off. As for the leather, the Piel probably has a little better quality. However, Sena's is very good. The Piel offers a little more protection around the edges of the bottom of the pda. Sena's case has more exposed bottom corners. Piel's clip is removable whereas Sena's is not. Piel's case offers more card slots. Since Sena's is magentic, it is advisable to not keep cards with magnetic strips on them in the case. As for me, I carry my credit cards separate from my pda.
Both cases are good. It comes down to your personal preferences. Usually, the Piel costs more than the Sena, but not in this instance. So, whichever one you choose, let us know how you like it.
