From what I've heard miss chance is much better than AC though i doubt that. In most cases though true seeing spell will suffice and having a constant etherealness spell would certainly cost. On the other hand a really high AC can be so much more helpful, especially a high wisdom monk/swordsage with some natural armor (feral template) works quite well.

Also are there any good ways to skyrocket your AC? Preferably without armor though

Ofc as there are ways to get past both high ACs and miss chance( for example true strike is a really easy way to skyrocket your AR)..So which is the best way to get some serious defence if you had to choose one of the two?

Miss chance looks very appealing since you can also avoid crits but it only works half the time since casters can dispell it and at higher level if you dont have permanent true seeing or something to help with illusions and the like then you're probably doing something wrong...Also wisdom bonus to AC(monk and swordsage stacks) works for touch AC too which is a great help against casters.