Should I choose tabs-in or tabs-out cassette shells?

That depends. In most equipment the absence of tabs makes it impossible to record over the contents of the tape. National Audio always uses tabs-out shells for duplication orders to prevent the tape from being accidentally erased by the end-user.

If you record on cassette tapes yourself, tabs-in are easiest, but you can record on tabs-out cassettes by covering the holes with adhesive tape. Breaking out the tabs (or removing the tape) after recording prevents erasure.

Some professional recording equipment can record onto either tabs-in or tabs-out tapes.