Both degrees are accepted by universities around the world. However, American universities have a better understanding of IBDP, and A-levels are the benchmark by which most universities in the UK set their course access requirements. To be honest, almost every university would now understand and recognize both. The best universities in both countries recognize these three curricula.
However, there is a level of familiarity that universities might have with their own country's curriculum. This doesn't mean you won't be entering American universities with A or IB levels, or vice versa. If you want to challenge yourself, the IB would be the most rigorous. Levels A, on the other hand, give you a level of flexibility while challenging you and allowing you to get the best score due to the structure of your exam.
If your school doesn't offer an international curriculum, taking part time AP classes can help improve your college admissions profile. All US universities accept the IB Diploma now and know all its requirements and value. Most US universities offer credit for IB courses (it's worth applying for), and many offer college credit for AP courses as long as exam scores are high. As mentioned above, some can also give credit for A levels.
That credit may allow an incoming freshman to skip the basic course of the 500-person survey taught by a graduate student, or reduce expenses in one semester (if a student has enough credit to skip 3 or 4 of the total courses required to earn a degree). The IB Diploma is an alternative to A-levels for students aged 16 to 18 and does not require any specific preparatory class work, so it can be done directly from the GCSE. It offers a broad curriculum composed of six subject groups: students usually study three at a higher level and three at a standard level. Both Level IB and Level A are school programmes for students who want to be well prepared to study in the UK.
While A-levels generally involve less work than the IB Diploma, the latter qualification is excellent for those with a wide and diverse range of academic, sports and creative interests. The IB incorporates work into projects (expanded essay and more) that helps students develop their research skills. Top universities will also recognize that success in the IB Diploma demonstrates the applicant's ability to take the lead in time management and effective prioritization. While Birmingham asks for four A grades from its A-level applicants, the University of Manchester only asks for three A grades.
Internal evaluation usually takes the form of long-term projects, such as papers, reports and presentations, accounting for 15-25% of the final IB score for that subject. For students who wish to enter British universities with an IB Diploma, it is important to know what the admission criteria are WHILE the student chooses their courses for the IB Diploma. Perhaps surprising to many British families, these universities clearly indicated that the IB Diploma was the best preparation for college and the workplace. In any case, it is important to consult the college and career counselor of the high school - this is the weak point of the case: many US councilors will not know about UCAS applications for UK universities, and British councilors may not know about strategies for applications US (scheduling SAT exams, teacher referrals, college visits and interviews, application deadlines, etc.).
For example, in the IB you can't avoid studying Literature or Language and everyone must study Mathematics (although there are more difficult and lighter versions) and at least one science subject. The fact that several universities in the UK interpret the IB curriculum differently makes the work of university advisors and IB coordinators a little more difficult. The IB is an international organization and its goal is to make its students know the whole world through the use of international resources and content. Similarly, for most UK teachers and professional advisors, the A-level curriculum ultimately shapes any guidance you give your students to prepare them for college and the world of work beyond.