Just to highlight how difference things are in practice compared to academic study, you're actually talking about two different areas of practice. Professional and clinical negligence are both tort, but distinct practice areas for the most part. Clinical negligence is associated with a personal injury practice, and is a practice area that you would often work towards rather than be doing from the start, though that can of course differ from set to set, and with the higher end sets that specialise in clinical negligence you may well find yourself doing it early on or from the start. It is also worth noting that the lower end PI work has taken a very heavy hit with recent changes to quantifying whiplash damages. That hasn't fully filtered through yet, but it will do by the time you're anywhere near practice. There are still residual arguments going on, as there always are, around certain aspects of the legal changes, but the landscape with PI is going to change drastically over the next few years, and as of now it's difficult to predict what form that will take. It may be, for example, that there are fewer PI pupillages but it's easier to access specialist areas like clinical negligence if you do get one, or it may be that your practice needs to be more diverse to compensate for the lack of lower end work. Either way, there is considerable uncertainty there.
By contrast, professional negligence is more associated with a more commercial practice, and in fact if you look at the top ranked Chambers on the Legal 500 for professional negligence, you will see sets like Brick Court. And that's because these are very often commercial or commercial adjacent disputes. An architect being negligent which has resulted in £500,000 of remedial work to a building is very different in practice to a surgeon who has been negligent resulting in someone being paralysed, even though they are both based in tort.
So the answers to your questions will vary depending on whether it is professional or clinical negligence, but generally, both practice areas will provide variety. Lower level personal injury can become very samey predictable, but clinical negligence is more complex and therefore, I suspect for most, more interesting. Professional negligence again has almost limitless variety, so there will be common themes but there'll almost certainly be enough there to hold your interest. The challenges will be like any practice in any area. The work will scale in difficulty as you get more senior, but will be challenging. Lower level PI, again, isn't all that challenging and things like Stage 3 lists can be done in your sleep, but clinical negligence is quite different to that.
On the complex point of law point, there are very few practice areas and practices where you will commonly be arguing complex legal points. Most cases and trials in any area of law may involve points of law, but fundamentally turn on the facts. It's no different in these two areas. Again, because you're studying cases which did turn on legal points you will naturally overestimate the extent to which disputes on the actual law plays a part in your day to day life as a barrister. In nearly all practice areas, the answer is not all that much, at least early on, and for many well into high levels of seniority too.
The extent to which you're in court will vary. Commercial practices are known for keeping barristers out of court more than other areas, so a practice that sees you doing professional negligence is likely to be more paper heavy than average. Clinical negligence will also likely be part of a more varied practice, so it will depend. But in either case I expect you'll have a good mix of days in court and paper days, unless you have a heavier and higher end commercial practice, in which case you'll be in court less. Certainly I know that Blayze has said that there are times when he might only be in court one day a week, or less frequently.
For pay, it depends on your definition of lucrative. Professional negligence is likely to pay more, but then those pupillages, particularly at the higher end, can be much harder to get. Clinical negligence is by no means easy to get into though. In both cases you should earn enough to live perfectly comfortably. If you have a decent level commercial pupillage you will likely earn much higher than average from a very early stage of practice, whether your practice involves professional negligence or not.
As a final broad point, all of these questions are understandable and there's nothing wrong with them, but part of this journey to develop yourself into a pupillage candidate is to bridge that gap between academic study and your understanding of actual practice as a barrister. You're not there yet, and no one would expect you to be, but it's just to note that this is a process that you'll progress with as you do more mini pupillages, marshalling etc, and actually gain that experience of what being a barrister is like. Threads like this are absolutely fine, but just realise that what you're studying academically doesn't necessarily map into whether you'll enjoy practising in a particular area. I do now practise in employment law, but didn't enjoy the employment law module at university at all. Equally, I quite enjoyed family law as a module, but from a very early stage it was obvious that I would never want to be a family practitioner. These things are always very different in practice, for better or worse, so over time you'll need to bridge that gap between the two in your own mind.