Red,
I'm going to start with an engineering answer, rather than an audiophile answer. Audiophiles tend to believe there is always one "best," and everything else must therefore be crap. Engineers know that there are always tradeoffs.
To summarize the speed tradeoffs: faster gives you lower noise and better transient response, or better HF response if you hear it that way. Slower gives you better LF response. There's also the obvious difference in tape cost, but let's stay focused on the performance differences.
To summarize the width tradeoffs: most things get better as you go wider, except that maintaining azimuth gets more challenging. This is not trivial. With wider tracks any edge damage waver becomes proportionally less compared to the signal. If there are minute dropouts they also are lower in proportion to the signal. The noise is reduced. And for some reason, which I can't explain from theory but I have observed, the head bumps become more manageable. However, unless your tape path is spot on you get azimuth wander, which leads to image wander. Ask Mike Spitz some time about all the work he had to do on tape guidance to get the ATR 1" to meet his (admittedly high) standards.
Here I will acknowledge that there are some, largely in the LA studio world, who prefer the sound of 1/4" over 1/2". Fine, everybody gets to like what they like. But since most of these guys aren't even mixing to analog any more, rather they are just mixing back into their Protools rigs like everybody else, I wonder if there might not be some retro nostalgia involved.
OK, now to summarize how I hear it: 7-1/2 IPS just sounds soggy to me, the reduced HF and transient information loss is more than I'm willing to put up with. Although I hear the HF extension and transient snap of 30 IPS, I also hear the lack of weight. With the right combination of transport, heads and electronics the HF and transients of 15 IPS are just fine. I think 15 IPS is the sweet spot. Not only have we chosen it for the Tape Project--both for our release tapes and for our running masters--it's what I choose for all my work. And what I bring home. Maybe you've never thought about this, but obviously I could make myself 30 IPS copies of the TP masters if I wanted, as well as other projects I work on, but I choose 15.
As far as width goes, I'm an unabashed fan of wider. I just hear more solidity, more depth, more "there" there. Except that when listening to 1" tape, it's only a matter of seconds before I quit listening to the details of sound quality and just get lost in the music. My shoulders relax, my eyes close, and I'm somewhere else.
