I prefer to paint man size GW models. Occasionally I'll paint a "monster" sized model but I just don't enjoy painting the large models. I usually choose an elaborate paint scheme, which just takes me so long on large models, see my Landraider for a case study. Consequently my heavy support choices used to consisted of heavy weapon toting infantry men for the majority of games I've played with my army.
That became illegal for 100% Nurgle armies, so slowly I've been painting vehicles. I've given rhinos their own page as I have three now. Although I can field Plaguemarine Havoc squads,their lack of guns that shoot over 24" means I rarely do. My total heavy support alotment consists of: three dreadnoughts, one defiler, one landraider, and one predator.
Although from my collection you may infer that I think dreadnoughts are best, I don't, I think they are the cheapest, predators are probably the best heavy support option. I planned to paint another one, but I'm trying to move on to Chaos Undivided, so my unassembled still in the box predator may end up in a non-Nurgle color scheme. Dreadnoughts, especially with all the conversion possibilities are fun to paint though they are a lot of work, especially the Forgeworld ones.