Healthcare Science has more extensive placements to enable you to complete the IBMS portfolio training you need to complete to work as a BMS, meaning when you graduate you can apply for full BMS jobs straight away. With an accredited regular Biomedical Science degree if you want to work in the NHS you would need to apply for a Trainee BMS post for which there can be a lot of competition.
There are some pros and cons to both.
On the downside of Healthcare Science when it comes to a dissertation its probably going to be something fairly practical e.g. validating some new test or analyser; while you get to state a preference for which specialty you go down, the university, the predicted demand from hospitals and grades are a factor too; its a bit more intense too with summer of year 2 until the end of year 3 being one long placement with distance learning. However, you completely bypass the need to find one of the limited trainee BMS posts, you can hit the ground running apply for any full BMS post straight away. And of course you get your tuition fees, a small means tested bursary and stipend, and your expenses while on placement covered too (though you can opt to pay privately if you dont want to commit to work in Wales for a couple of years).
Biomedical Science is going to offer a bit more freedom for the dissertation which might be of interest to those who are looking to use the degree as a route into Graduate Medicine or some other allied healthcare profession as you'd potentially be able to do some relevant research. You're also probably not going to be quite so thoroughly dictated to on what specialty you have to do.
In summary Healthcare Science is Biomedical Science + IBMS Portfolio which you actually need to do the job. If you've got the choice and don't mind working in Wales for a couple of years after graduating it's a pretty good deal.