The earlier reviewer is quite right. I first acquired these two symphonic performances on a budget Decca LP reissue many years ago. When I got hold of the Dutton reissue on CD, I was amazed by how well the recordings had come up. They were from the days of Decca's pioneering "full frequency range recording" mono technique and the First comes over extremely powerfully - maybe the colours are primary and lack some of the tonal finesse of a really good modern stereo recording, but for something over 50 years old it's nothing short of amazing!
The performance of the First Symphony strikes me as one of the very best, if not the best, that I've ever heard. It has tremendous forward drive without seeming in any way hard-driven. The Fifth is probably the greatest of Nielsen's symphonies (though some might argue for the 6th) and this performance is indeed excellent. The only one to beat it, in my view, was a live performance at The Proms given by the Gothenburg orchestra under Neeme Jarvi (unfortunately Jarvi's recorded performance on BIS doesn't come close to it in grip or atmosphere).