Generally, the higher the tape speed, the better the sound. Also, two-track machines have more head space to work with so usually, better sound.
Pre-recorded tapes from 50 years ago were good if they were 7.5 ips and barely better than a Dolby-encoded cassette at 3.75.
Working with R2R tape is a lot like having a turntable. There's something very satisfying about looping the tape around the pressure guide, past the heads an over the capstan, etc. The TEAC decks mentioned are among the best you can find. According to the guy who restored my Akai GX-620, ReVox A77's aren't all that reliable and when they break, they're a bear to work on, thus, expensive. However, they sound really good when they work.
These $10k play-only 15ips decks are grossly overpriced IMHO. If you want to go that route, find a used Otari 5050 or Tascam 32 and save yourself several thousand dollars. Both are reliable, sturdily built and sound fabulous.