Bandos is power armour which let's you deal more dps, while having lower armour values. Eldar rune is tank armour and helps absorb some damage. For most things power armour is the way to go. I would recommend not switching out your armour.
Depending on what you are actually doing accuracy is generally more important than damage. As better accuracy leads to more dps if you don't have 100% accuracy. Your dragon rider Lance has t90 accuracy with t80 damage. If you want to upgrade that weapon I would recommend a nox scythe. Also it should be noted you have a 2 square range with the Lance that you would not have with the eldar rune 2h. It helps a lot in many locations.
Short answer: Don't replace either with eldar rune.We are all made from the same thread.