Each promotion brings different gear with different stats. Some promotions are great for some heros & actually make others less effective. Runes is a way to counter that & make up differences but unfortunately you guys don't have those. Best thing is to talk to other players that have promoted those heros & see what they think of them post promotion before promoting yours. Sometimes (even with runes) some heros are better left not promoted. The first noticeable difference with this was promoting from p4 to orange. Quite afew heros were better at p4 than they were orange & as word got outmany players would wait & skip orange & go straight to o1.
Another thing is to wait until you have gear to promote to the next. The first 3 pieces of gear carry the best stats in each promotion & make the biggest difference in power also. With the blue series it was best not to promote until you had the first 2 pieces of gear for the next set. With purple the first 3. With orange through o6 the first 4 gear. And with o7 & o8 all but the last one. If you promote too early you actually hurt your heros by promoting them.