If it were me, probably Valenca (just across the river from Tui, in Portugal). Others have mentioned it being more crowded from Sarria, but it isn't just that. It is who will be with you. The larger groups that also start from Sarria and the pilgrims who have walked much further and already formed their "pilgrim families" may make it a little harder to connect with others (although certainly not impossible). As well, unfortunately some of the long distance walkers will be seeing you as "Johnny Come Lately" to the pilgrimage. You will likely find the percentage of pilgrims who judge you by how far you have walked lower if you start from Tui/Valenca.
In addition, it is a lovely route through wine country and takes you past some locations with the closest historic or legendary ties to Saint James (whom you are walking to see, after all). And if you start in Valenca, you get to see a bit of Portugal, as well.
From the fact that you've posed the question, I'm assuming this is your first Camino. It isn't a certainty, but most people walk the Frances or the Portugues first. In that case, if the experience whets your appetite for more, you could consider returning to walk the Frances in the future.
Any advice will be truly appreciated.
