Former lawyer here.
I didn't do APs; I did my country's A Levels instead, which is probably at least comparable, if not more difficult. Suffice to say, Oxford is absolutely a step up from A levels, and Law Finals was probably the most difficult exam I've ever taken. The difficulty stems from a mixture of pressure, lots and lots of content to understand and memorize, and needing stamina and mental strength to keep going for ~2 weeks. The first week, where you take 6 papers back to back Monday to Saturday, is brutal.
That being said, as the Faculty likes to note in its handbook, it is very much a baptism of fire, and most people do just fine. Bear in mind that the vast, vast majority of people (~70%) will end up with a 2i, which is more than enough for most jobs, including being a solicitor. The real challenge comes if you want to go for a First, but even then, the % has been steadily increasing (something like 20-25% now I believe, which is high compared to other UK universities). You can read past examiners reports here:
https://www.law.ox.ac.uk/document-archiveI will say that in general, UK universities tend to base degree classification on a handful of exams/ assessments, most of which are taken at the end of each academic year (eg Cambridge). The only real difference is that Oxford combines two+ years worth of content into one big exam. Cumulative assessments in the sense of a GPA-style system aren't really a thing. If you plan on coming back to the UK to do a law degree (whether a 2 year senior status or the full 3 year), you probably can't avoid this "do or die" system entirely.
The other thing to think about is finances. 4 years at an Ivy (not to mention the high living costs of NYC!), on top of a further 2/ 3 years of law school either in the UK or the US would add up to an awful lot of money. You might be able to get financial aid for Ivies, but UK universities in general almost always never offer financial aid to undergraduate international students (with a few exceptions, like the Reach Oxford Scholarship). If you subsequently wish to go on to graduate studies (eg BCL/ LLM) , that's additional time and money as well. Remember to factor in any relevant professional legal qualification programmes too, if applicable (eg LPC).
If you're a good test taker, the Oxford system probably favours you - it very much means that you don't have to go all out for 3 years (most people do tend to slack a little in second year as there are no exams), whereas you have to be consistently good in a US university. It also means that if you do take some time to get used to the pace and content, you won't be penalized for it because the "teething stages" don't count towards your classification. Mods is only 3 papers, which I imagine is very doable considering your history in test taking.
You didn't mention your post-graduation plans - where are you intending to work? If it's back home in the Middle East, you need to check the relevant qualification procedures and degrees (eg my home country does not accept most US JDs). If it's the UK, assuming you went to an Ivy, you can actually just do a 1 year GDL instead.
I honestly believe that it's up to your personal preference, horses for courses and all that. I've never been to an Ivy, and so cannot comment on what education there would be like. That said, I do think Oxford provides a superb legal education; there's a huge emphasis on really thinking from first principles and evaluating the law which you don't see as much in other law schools (barring maybe Cambridge).
Out of curiosity, are you holding on to a deferred offer for the Ivies or did you get a likely letter? It's been a long time since I applied, but I thought RD decisions are only released around end March/ beginning April? Columbia doesn't do EA as far as I know, and ED would be binding.