The AHCS equivalence route is self-directed, so you essentially have to prove that your experience is equivalent to the STP. This means that there isn't competition as such, because you could theoretically get equivalence at any time. This might seem like it's easier but in practice it's much more difficult because you need to show that you have the equivalent knowledge to a masters degree in a specialist area, extremely relevant lab experience and a thorough knowledge of how that area works. This means that it's almost entirely biomedical scientists in a specific discipline who gain equivalence to the STP in the same discipline. At least in biochemistry I'd say that currently well over 90% of clinical scientists come through the STP route (either as in-house or direct applicants) and as an "out-sider" to the NHS the STP is probably the easier route in, despite the high competition.
Once you've completed the AHSC route then you are considered identical to someone who's gone through the STP so would be free to apply for HSST if you wanted to. I would say that (again, at least in biochemistry) not many people currently do the HSST and it's not currently required to apply for consultant jobs.