I'm a graduate currently training towards becoming a CA with a top 10 firm and we have a number of apprentices with us so I'll try to answer from the best of my experience. In terms of job opportunities after qualifying, degrees become almost completely superseded by being a chartered accountant. By that stage if you have your accountancy qualification that is the main prerequisite for any job and from there the most important factor is your skills and experiences. Put it this way; the job market after graduating university is tough. The job market once you have a CA/ACA and years of experience isn't. You can (within reason) almost pick your job (and on a very good salary) - after university it's likely you'll be happy to take any job you can get (and on a fairly low entry level salary).
Starting salary for apprenticeships is less than graduates (generally £12k - £19k compared to £19k to £27k, dependent on location). However after qualifying you'll be on the same as any other newly qualified regardless of their route (roughly £35k - £43k, again dependent on location). Bear in mind you'll possibly be younger than the graduates at that stage too, and this works hugely in favour of school leavers who wish to work their way up in the firm; a large proportion of directors and partners came straight from A levels and never went to university. You will also have more years experience in your training which is only a benefit, and it also makes the studying process slightly easier as exams are more spread out. It's also a nice feeling being paid to go to college/study whilst knowing you could be in university being saddled with debt!
The only potential career drawback is that some companies (especially banks) still require a relevant degree but it's not especially common and if you wish to continue in accounting practice you won't face this barrier. Also, if you wish to work abroad after qualifying you may find yourself slightly more restricted than a graduate as many countries require you to have a degree in order to qualify for a visa.
Overall there's not really too much downside at all to an apprenticeship in accountancy and had I been aware those opportunities existed I probably wouldn't have gone to university.