Thank you for the detailed response, lots of things to think of.
I think the performance standard of both LTCL/LRSM are equal. The thing that makes LRSM more challenging is the supporting tests. Although in either you should aim for the best you can possibly give, I've always had the impression that Trinity are slightly more forgiving of slips at this level than the ABRSM. But that is a totally subjective comment that shouldn't be given too much weight in the decision.
You make a good point with approval. Although the LRSM syllabus says prior approval is not needed for own choice items, I'd still rather not be penalised if they thought The Carousel Waltz wasn't on par with the rest of the syllabus. LRSM specifically says "one, two or three of the 6 Images", LTCL does need clarification I think. They are the item on my program that I refuse to budge on, so if LTCL only allow one, then that's my decision made!
The difference in playing time between the two exams is miniscule. Trinity want 37-43 mins, ABRSM want 40 mins plus/minus 10% which works out 36-44 mins. The Carousel Waltz is about 6 mins and my Images take around 14-15 mins as I allow the first two pieces a bit more space than most pianists. My hands-down favourite Debussy interpreter, Noriko Ogawa, plays them in 15'30. So I'm near enough smack on 40 min total.
There is a performance of The Carousel Waltz on youtube, but it's absolutely awful in my opinion. Because you have been so kind and helpful, I am happy to share with you Hough himself playing it if you're interested!
I agree, the examiners will have good understanding of the majority of the pieces on the syllabus so it's probably not important.
One strength of LRSM is that it says "aim to present a balanced programme that includes a contrast of repertoire from at least two distinct musical eras". I hold my hands up and admit that I am not comfortable playing Baroque and Classical repertoire. Not a matter of taste, just the knowledge that listeners are far more judgemental when listening to music from that period. Would I theoretically be able to build a program of Romantic and 20th Century music for LRSM if it was contrasting enough?
I have wondered the same thing re sonatas, but my take on it is that although it may be a preference, surely it wouldn't altogether put them off a well performed recital?
I could do FTCL with a bachelor degree, but not FRSM. I don't think I am anywhere near such a level of performance at the moment. I am considering applying for a performance masters after passing this diploma using the same pieces, which will be the time and place to begin work on such high level repertoire.