I've only seen 3 3/4 ips.
I can't tell you how often I've seen old tapes for sale and thought, "I gotta have that one!" and scrolled down the ad and seen 3 3/4 and been bummed. Dylan, the Stones, Hendrix,.. Hendrix's ( I have to brag) " Are You Experienced" 7.5 ips is on it's way to me now! Of course, the proof will be in the pudding but it's the first one I've seen at this speed.
Some rock tapes have been produced at both speeds. But some of the European/British dubs don't sound real great. I think that MCA/Warner tapes come in two flavors and the lime green labeled ones may have IEC eq.