HI
@Ahaj
Last October I walked from SJPP to Roncesvalles for the 4th time. All previous times I took the steep path, but this time I took the alternative. I thoroughly enjoyed it and found it picturesque. I've read descriptions of it as 'walking on the road' but I think that's misleading. It's a very quiet 'road' and much of the time we were on a grassy verge. I wonder if that was your experience. There were some lovely views looking down to Roncesvalles and I liked the entry into Roncesvalles.
The steep path through the forest is atmospheric for sure, but I seem to recall that each time I've walked it I was very focused on how I was walking, and being careful not to trip or slide. I found the alternative more enjoyable in that regard. So, in trying to answer your question, I have enjoyed both but I too have a few knee issues - and if I walk that path again, I will choose 'the road' option again.