Hi Algeeba, Welcome to the board!
I'm actually going to go another route completely here and suggest that you consider playing your tapes into the line-in on your PC and using WAV recording software (like soundforge) and burn them to CD, rather than making tape to tape copies. It might be a bit more work for you, but I think you'll have much better recordings that way
If that's not an option for you, I'd go find the best looking used Maxell XLII's you can find in thrift stores or maybe an ebay seller getting rid of his Greatful Dead concert recording collection. Those used chrome tapes will do a much better job than brand spankin' new Maxell UR any day of the week. I have bought a great many of those used tapes and had awesome results with them. Another very good, and plentiful tape out there is the TDK SA. They are just as good as the mentioned XLII, and depending on what year they were made, they might even be better.. Last week I bought a dozen used XLII's at a thrift store for $0.20 each.
If you don't mind me asking, what are some of the titles you wish to copy? Are your cassettes really rare, is that why you don't want to use them? Unless your tapes are exceptionally rare and somehow sentimental to you, I would not really bother to make backup copies of any prerecorded tapes. I say all of this this because a majority of the prerecorded tapes I've heard sound flat and lifeless when compared to the original vinyl or CDs. I own over 500 tapes with music on them and only 30 or 40 are prerecorded. The rest were all recorded by me.
In your place, I'd consider hitting thrifts, flea markets, garage sales and maybe even ebay to source the CD's to match your cherished tapes. You can buy them for pennies on the dollar vs. what they cost new nowadays. Even eBay is a good option. Then you don't have to make any copies. And if your tapes get eaten by an aggro-tapedeck, you can make a new tape using your purchased (or burned) CD and put it with that tape artwork.
As mentioned before, I found that I can make a better recording than 99% of the prerecorded tapes I've ever heard. No joke. Using XLII or TDK SA, just set your recording levels to have your VUs peak at around +3 to +5 db and I think you'll actually find that your recorded copy sounds better than your prerecorded tape ever did. If using a UR, set your peaks a bit lower as that tape can't handle as much signal as a quality chrome, like to peak at +1 instead.
Making tape to tape copies is always a bit of a challenge and your copies will seldom (if ever) be very close to the quality of the original source tape. I don't know how good of a performer your deck is, but often the sound quality drops two complete letter grades from original tape to the copy, especially when using the lower end of available dual well decks. And if you use the high speed dubbing option that will typically make matters even worse still. The exception occurs when you use top shelf tape decks and employ the use of an equalizer between them to shape the sound to your liking. (As Stranj hinted, I have a thing for Nakamichi decks. I happen to own four of them).
But, you should definitely experiment if you have some time on your hands and want to learn something about tapes and recording.
Do you own any tapes where you also have the CD??? If you do, then I suggest you take an hour or two and make some comparisons... This exercise will help you decide exactly how you want to go about this project. Get the matching CD your prerecorded tape and pick a song with quiet passages and some strong dynamics... Then, go to work and try these methods:
1) Tape to tape copy onto Maxell UR
2) Tape to tape copy onto Maxell XLII
3) CD to tape copy onto Maxell UR
4) CD to tape copy onto Maxell XLII
5) Tape to PC copy and burn to audio CD (IMPORTANT when burning into PC, make sure you stay below -1db or -2db on PC's VUs at all costs, check your levels during the loudest part of the CD or tape or you will get digital clipping which will ruin your recording. Digital recording cannot handle nearly as much signal as analog recording)
I think you will find that #4 will be the clear winner, which is probably also the most expensive option. #5 is the cheapest solution, but you may need to buy software to record WAV files and burn the CDs. (I recommend soundforge for recording and CD architect for burning CDs. you can get old versions cheap or free if you are clever)
Of course sound quality is entirely subjective. And if you can live with the quality that option 1 gives you, that is by far the the "easiest" overall solution as it just involves simply popping in tapes and hitting the dub button. You can forget all about what I've written and go forth with your project as planned.
As for whether to use 60 min or 90 minute tapes, that depends on your sources. I would say try to have both on hand. Most older albums would fit on a 60 minute tape, but when CD became the standard, artists started releasing albums with longer and more tracks to fill up the 74 minute redbook standard CD length.
Good luck and have fun! And ask questions if you get stuck... There are lots of guys here who are just as experienced at this as I am...