When to use obsidians vs Blazes?

Trying to figure out the best choice when using the two would like some insight... Thanks

In my humble opinion, Blazes are great when you get tech 2 rather quickly. They're relatively cheap yet very strong against tech 1 and they can evade fights that they can't win by going over water. Other factions do not have an immediate answer to this as their hover units are usually a lot more expensive or weaker. Obsidians are rather slow, but have high alpha. They're great when you need to defend in my opinion, they're not so great when attacking because they are so slow.

99% of the time I'll go Blaze because they're easy to spam out like T1 and just overwhelm. If you get to a point of blaze spam not working and needing obsidians, then you're probably better off just going T3.

I agree with Jip's Blaze-take;
for the Obsidians: Since Aeon has mobile shields and the best midgame-ACU, Obsidians can be used as a late T2 / T2.5 spam instead of going T3. I personally also use obsidians if I fight against multiple ACUs or want to crush multiple bases, going for an all-in with my push. You have to make sure to not get overrun by T1 spam, so mixing in some blazes in the beginning is usually a good call, with the additional T2 PG you can also go for shield and Overcharge regularly with your ACU.

A different explanation for Blaze is just the Aeon variant of the Pillar.

Obsidians are ACU killers: they survive overcharge, their high alpha strike is not a weakness against fat targets, their slow speed is still faster than that of an ACU. You see an enemy ACU going too deep into your territory? Accumulate some obsidians are run it over.

I use Blazes in early T2 for their range, and Obsidians to punch upward at stronger stuff. If you're falling behind and in a pinch, Obsidians can get work done.