I've bought quite a few albums on 33 and then the 45 version when available. These are with various labels, but my comparison is generally within a particular label. In all cases I've found the 45 rpm to be significantly better. Any album that I really treasure I will always seek out a 45 if it's available.
33 vs 45 speed quality
I just got into analog recently. My TT is a rega P9 which blows away my cd player for music quality. But...in truth I cannot tell any difference between 33 and 45 speed recordings. Is it just my ears or is my setup to Lowfi? I am using a Mac 6300 integrated amp and audiophysic spark speakers.