Hi, Joe,
On the Primitivo, there are no climbs that even come close to the day out of SJPP (which has 1100 m of elevation gain). I just checked the elevation gain from Castrojeriz up to the meseta, and it's only 81 meters! I think it's just the steepness that makes people huff and puff, not the total elevation gain. And I don't know the Vasco Interior yet, so I can't comment on that one.
I remember that way back when I was deciding to walk the Primitivo for the first time, people were saying it's the toughest camino, lots of elevation. I didn't find it to be unusually tough, but I know that's a personal thing, so I thought I'd get the facts of elevation gain (and I'm assuming Mundicamino knows what it's talking about).
There are two significant ascents on the Primitivo. One is up to the Puerto de Palo. This can be done one of two ways -- from Borres up on the route called Hospitales (goes by ruins of several pilgrim hospitals, is remote and has beautiful views), which has a 400 m gain. It's gradual and very manageable. The people in the albergue on the Norte in Pola de Siero had scared the living daylights out of three young people I later met up with, repeatedly telling them not to take the Hospitales route. It is all hype; these young folk felt pretty silly after we arrived on the top .
The aternative way to reach Puerto de Palo is to go to Pola de Allande and start from there. This alternative gives you a shorter distance to the next stop (usually Berducedo) but more elevation gain. From Pola to Puerto de Palo is 600 m gain. It's a totally different walk than Hospitales, it is mainly through beautiful green tunnels, lots of babbling brooks, farms, etc. I've done them both and they are both wonderful.
The second ascent is from Grandas de Salime to Puerto el Acebo. That is 500 m total elevation gain but it's gradual.
What the primitivo does have is two unpleasant descents. One is the 800 m descent down to the dam and reservoir right before Grandas de Salime. The other is the 400 m down from Puerto de Palo to the village of Lago. You can get an idea of steepness by looking at mundicamino.
The other steep ascent that people remember is on the stage between Fonsagrada, right before A Lastra. This is primo mud territory. When I walked it in June, it had been raining all night, but the rain stopped at about 6:30 a.m. Many people took the road, which is several kms longer and a lot less prettier. Several of us decided to take a try on the camino. The first segment, from Fonsagrada to Paradavela, was fine. There is a bar right on the road at Paradavella, located just before two of the more potentially muddy segments. The day I was there, the bar owner regaled us with warnings about how awful the camino would be and he urged us to take the road. Given the proximity of this part of the camino to the highway, several of us decided to take our chances (reasoning that we could always backtrack if things got too muddy), while the others stayed on the road. The camino was a little muddy in places, but totally passable and just beautiful, through a few abandoned hamlets. Right before the camino rejoins the highway in A Lastra there is a steep ascent. I can't estimate the elevation gain, but I timed it and I can tell you it took me less than 20 minutes. Point of this rant -- do not trust the word of the bar owner in Paradavela. He's a nice guy and means well, but he has never set foot on any part of any Camino. Go check it out for yourself. It may be muddy and impassable, but you can decide on your own. One Spaniard I walked with, who has walked the Primitivo upward of 10 times, told me that his rule of thumb for this section was that unless it is raining very hard at the moment he leaves Paradavela, he sticks to the Camino. If it is raining hard, he either waits for the rain to let up (the water drains off very quickly, leaving gobs of mud but it's not that clay mud from the meseta) or reluctantly takes the highway.
I can't give you a poles recommendation because that's another one of those very personal things, but I use two poles and wouldn't take a step on any camino without them. Many others disagree.
I will hope for great weather for you, too, you are going to love the Primitivo and its incredible scenery, wonderful albergues, interesting cities/towns (Lugo is stunning), and just the right number of pilgrims.
Buen camino, Laurie