Tough question! I've done the Frances from St Jean de Pied de Port in 2008. This is a great route as the scenery changes. The big issue with the Frances, and something that the 'start in Sarria crowd' don't experience is the 'meseta' the bit between Burgos and Leon. I LOVED this bit. It had wide open spaces and a sky that just didn't end. The flatness was awesome. Other people, however, regard this bit as terminally boring.
I've also done the Portugues from Porto. I agree that once you get out of Tui (hopefully the sign posting has improved from 2009) that this is a pretty route. You also get to go through Padron, home of the famous pimentos de padron (not to be missed). One of the highlights is walking through vinyards under pathways covered by grapes.
I did the Ingles in 2010 for the Holy Year. This is a good route for people who want to do an entire camino in 5 days, but it isn't as scenic as other routes.
This year we did the Primitivo. If you start in Lugo, you can get your compostella as Lugo is 104km from Santiago. This was very secenic. Lugo is a great town. You have an amazing Roman wall to walk around (over 2km and a world heritage site to boot). Old Lugo is great and has amazing tapas bars. See my 'live from the primitivo' post for more info about this route and Lugo. Leaving Lugo, the route stays mostly on tracks or nice and small country roads. From Lugo there are some really wonderful albergues in San Roman and As Seixas. You have to bring your own food to these places. You hit the Frances route in Melide.
If you only had 6 days and wanted something scenic and not too crowded and were prepared to rough it a bit and carry a few days food (pots and pans and cooking facilities are provided) I would do the primitivo from Lugo. That being said, if you have more than just 6 days, there is a lot to be said for starting in Astorga. Rabanal is a great place to stay. You get to go up and past the iron cross (this is where you leave something of significance), you get to meet Tomas, the self-proclaimed 'last of the Knights Templar' and a very nice guy (a true camino experience) and then you drop down into Moulina Seca which has some great swimming in the river. I really enjoyed this bit of the Frances, but if I had a choice between starting in Sarria (the last compostella possibility place on the Frances) or Lugo, I would go with Lugo. But then, I am biased by the experience of a few caminos. If you were only every going to do this once, you might as well toss youself onto the the not so tender mercies of the Frances and get the full on perigrinoo experience.