I don't know the answer, because I did upgrade to the C64 during 1987, when my Spectrum 48k broke, after serving me well for almost four years.
Perhaps this topic could be labeled "UK only", because the rest of the planet had no such dilemma.
However, it is interesting how some similar arguments of French Amstrad fans are now becoming quite acceptable, while they seemed strange to us before.
But that's perfectly fine.
I'd probably think the same myself, if I lived in the UK, but I grew up in ex-Yugoslavia, where both computers were almost equally popular, Spectrum until the mid-80s, but after that the C64 became extremely dominant until the early 90s.
In my case it also coincided that two of my good friends already owned a C64 at that time, which was the deciding factor for me.
Sometime during 1989, a friend got a Spectrum+, but I was quite disappointed with the software support, too many new games were just rushed monochrome ports, while the C64 had more and more mega hits, including excellent Amiga conversions, which convinced me that it was a valid upgrade.
The next step was the Amiga 500 in 1993, and after that in the mid-90s, I went back myself to the Spectrum and C64, through the early PC emulators and from then until now, I've never had to choose just one model.
To summarize, Spectrum and C64 were never real competitors for me, because they are so different in their essence and conception.
C64 was, roughly speaking, a good gaming console with a keyboard and the Spectrum was an excellent entry-level computer, exceptional value for money.
Millions of users have made them legendary machines, I won't do either one a favor by underestimating the other.
In terms of nostalgia, I always give Spectrum the edge, probably because it was the first and as time passes, it always seems to me like wine - the older the better.
Standby alert 
