They are both very nice and both have sufficient infrastructure. The route from Sarria will be more crowded. I give a slight edge to Tui.
1) I think the towns/villages you walk through are more interesting: Pontevedra, Caldas de Reis with it's hot springs (or the beautiful scenery of the route of stone and water on the Variante Espiritual if you miss Caldas de Reis), and especially Padrón with its connections to St. James.
2) My recollection is that walking north into Galicia through wine country was nicer on the nose than walking west through cattle country
(although, to be honest, I don't remember how much of the cattle country was between O Cebreiro and Sarria as opposed to Sarria and Santiago)
3) This way you will still be able to say you walked across Spain from the border to Santiago
.
Really, it is the first reason that is driving the recommendation. Although, if you decide to start in Tui, you might reconsider and start on the other side of the bridge in Valença and experience a bit of Portugal.
Pros for starting in Sarria are experiencing some of the classic Camino moments: the climb up the stairs into Portomarin, with its fortress-like church, and the walk through the arched tunnel past the busking piper into the Plaza Obradoiro in Santiago.