Slipstream has different challenges than transwarp. For one thing, it's not well-suited to the shape and size of a Borg vessel. It requires very intensive computations to keep a slipstream conduit active, and the larger the conduit needs to be, the exponentially more difficult it becomes to keep it stable. (According to the explanation we worked out for the books, that is.) Hence the reason narrow ships work better.
In theory, it's possible to refine slipstream tech to handle bigger ships, but the Borg aren't innovators, just imitators. They assimilated Arturis's people's slipstream tech when they assimilated the civilization, but that drive was optimized for ships much smaller than what the Borg use, so they couldn't make practical use of it. And the transwarp drive they already had served them well enough.
Whether slipstream is "better than transwarp" depends on how you define "better" and on how you define "transwarp." We've been shown multiple different drive systems that were called transwarp, from something that seems to function like warp only faster to something that relies on pre-existing conduits. It seems to be a generic term for drives that are faster than standard warp drive. Maybe slipstream could be called a kind of transwarp if it didn't already have a distinct name of its own. As for "better," it depends on what parameter you're using. Slipstream is problematical in a number of ways. It can easily destabilize if your computers aren't fast and precise enough. It depends on a very rare type of synthetic crystal. It's useful only in certain types of ship. But, unlike the various kinds of transwarp, it's a drive that the Federation has actually been able to make work, and it allows for speeds that equal or surpass most kinds of transwarp.
