This is a tough one again. I like both songs very much. Here's some reasoning.....
Blackmores big solo on the Made in Japan version of 'Child in Time' was iconic for me, and coming hard on the heels of the big solo in 'Highway Star' on that same album made the 12 year old little guy who was me at the time think this guy was some kind of Master of the Universe come down to earth to share his skills with us mere mortals. I gotta say however that I'm not so keen on the 'In Rock' version of CIT as Gillans vocal screaming isn't as good as it was on the live version. It just didn't have the same knock you down sort of impact that the Made in Japan version had.
Stairway to Heaven is so bloody good and so completely timeless that it still brings a little tear to the eye on the odd occasions when I still hear it and I have a particularly special memory of that song from a house party one night in the mid 80s when I was jamming it on acoustic guitars with a mate and everyone in the house sang the lyrics right through from start to finish - guitar wise it wasn't particularly well played but the song magic was definitely there...a very fine moment in history for me....
Having said that however, I also have a clear memory of watching Led Zep perform at the Atlantic Records 25th birthday party celebration in 1988 (think I might have mentioned this before) and Page was either so pissed out of his head or so out of practice guitar wise that he made a complete arse of the solo. He literally couldn't play it. It was so bad and so embarrassing that I was blushing in shame for him and the fool he was making of himself up there on stage but fortunately most people didn't seem to notice. Believe me it was
bad though. It might be kicking about on youtube somewhere as it was a big event at the time....?? That was a defining moment for me Led Zep wise and they've never really redeemed themselves in my estimation since then, but I
DO love this song very much, so I guess if I was put on the spot and with all other considerations notwithstanding it would be "Stairway to Heaven" for me... Phew....