Hi Bjørg,
I walked part of the Southern Route in May 2007. I started in Verin and walked one day to Sandias (albergue) and the next to Ourense (albergue). Distances were 41km and 33km respectively. The first hour or so out of Verin is on the old main road (N525) so can be busy but verges are ok. Route then veers away from the road through some nice villages and with a couple of steepish but relatively short climbs, occasionally crossing back over the main road or the autovia. Once you get on to the plain that surrounds Xinzo de Limia the route is flat, straight and always close to the Autovia.
Sandias to Ourense again starts on the main road for several kms before cutting through lovely wooded paths eventually ending up in Allariz (picturesque town) and then via back roads towards the outskirts of Ourense.
I believe both Xinzo and Allariz have hotel\hostal accomodation so the distance could be broken into smaller chunks over 3 days (32.5, 21.5 and 20 kms respectively) hence giving a bit more time for "sightseeing". We saw no other Peregrinos on the route until we got to the Albergue in Ourense.
I can't give you much information on A Gudina to Verin other than it would be generally downhill and again probably criss-crossing the Autovia and\or the N525.
I've not walked the Northern route but have driven around that area and I would say that in terms of scenery it is probably the nicer of the two although as indicated by others, more isolated. The bar at Albergueria is definitely worth a stop.
If you use Google Earth at all, I posted an attachement in the following thread,
camino-mozarabe-and-via-de-la-plata/topic3745.html, showing the Verin - Sandias - Ourense route with albergue, sello, bar/restaurant information which you may find useful should you go that way.
Regardless of which route you choose, I'm sure you'll have a fantastic experience. Post back if you require any further info.
Bon Viaje.
Mig