I think p.26 is easier than p.11. If I'm very unlucky I may die only in 26.1 (remember there's 26.0 as well) if the

are mega-stomping. But if the

become too fast there are two levels I could die in p.11 plus the last one - the strawberries are at the two corners and there's a pineapple as well... 26 is easier for me.
If you ask about which p.11 or p.13 gives more points I'd say p.13, because you have more levels and more enemies there. Which means - more and bigger crystals.
However if you go in p.13 you will skip parallel p.15 and you won't be able to go through all those levels, you will miss many items, score and letters. So I always go in p.11 and then in p.15. I don't die in p.15, but I don't go in p.23 any more, because those 3 levels are MUCH risky than all those levels in p.15.