A bit late to join in the debate as it looks like the debate has finished, but here is my pennies worth.........
I was 17.5 stone and am about 16.5 now. I started cycling last summer and used a huge/heavy old hybrid with a triple - more mtb than road. I never did any hills and hardly ever used the granny ring if at all. I then bought a Boardman hybrid late last year (compact) and started to do more than just my 19mile flatish commute - I struggled a bit on the hills (and there are lots around the west of Scotland). Whilst I have never yet been defeated and had to get off, the speed I turn the pedals and move forward at is unbelieveably slow - I am sure I would be more comfortable with a triple.
Earlier this year I bought the Verenti Millook - thinking that a lighter bike, an improving/fitter me would result in not needing a triple. Again, I have conquered every hill to date without getting off (including the Trossachs Ton, but was probably the slowest man on the hills - although pretty quick on the flat/downhill), but it remains hard work and extremely slow.
IMHO I think with hindsight a triple would suit me more and would be slightly easier. I live in the hope that if I do get down to 13 stone then the compact will be fine - but its a hard slog in the meantime. With hindsight I'd go for a triple.
Incidentally, I did go up from Aviemore last week from the bottom to the ski-slope/station using my MTB (the Millook has gone back with a fault) - I am not sure if this is steeper than anything else I've done, but it certainly felt like it and the triple made things more bearable (just). Anyone who does this gets a free ticket to go up the mountain on the funicular railway - saving £10. I'll do anything for a freebie................