What's the difference between 33 and 45rpm?
Basically, it is similar to MP3 sound quality issues in the digital world. MP3 audio files encoded with higher kbps options (192, 256 or even 320 kbps) will sound significantly better than the ones encoded with lower kbps options (128 or 160 kbps). The reason is that the MP3 decoder can benefit from more sound information per second stored in the higher kbps MP3 files. Vinyl records (and magnetic tapes as well) sound better if higher speeds are used. The higher the rpms, the more vinyl that passes under the playback stylus per second. Therefore, we gain more accurate reproduction of the sound stored in grooves.
Scientifically speaking, the longer wave-lengths, the smaller angles in the grooves and the less complicated geometry at 45 rpm help us to cut very precise grooves with even the finest details. Also, the conical and elliptical playback styli track grooves better at 45 rpm – resulting in less inner groove distortion (pinch effect distortion, horizontal and vertical tracing distortion etc.). Due to the higher rotation speed, the available playing time per side is reduced at 45 rpm which often results in double albums, e.g. 45rpm audiophile 2LP editions with up to 15 minutes per side. For the best possible sound quality, we recommend ordering records cut at 45 rpm.
Please also read this article for the track duration available for each speed.