I'm in the same boat as you, I like trample better than speed...so I'd go Heavy D and save heirlooms for something else.
If you want a fast or maneuverable horse, dont use the destrier.
Ok but what is worth heirlooming now? Looking at the weapons I don't think it's really doing any difference (like 1 speed and 3 more damage for 3 heirlooms O.O'') so armors and horses look like the most worth to heirloom
