when i was below lvl 5 i have read this article in the web that says astronach is much better than undead because of their speed and doesn't require dead bodies to conjure. but then after i reached 80conjuration i have bought summon demora lord which i think is very awesome. he's so powerful compare to my astronach.
so i wonder if later in the game which summon will be much useful? demora lord, astronach, or thrall? ive heard undead thrall can equip staff that can further summon their own minion.
in the end i have these choices:
1. demora lord
2. frost astronach
3. thunder astronach
4. undead thrall
5. elemental thralls (astronach)
i hope i didn't wast my astronach perks for nothing :S

also for the destruction spells,
ATM i have fireball, flame, spark, frostbite, flame & frost cloaks, and chain lightning
are these good? in the later part of the game which spells should i get?

Get them all.

In the end you should be able to summon 2x deadra lords without significant magicka loss, which renders the entire game super easy. My mage was built around low health and conjuration, and near the end the only option i had was to summon something OP whenever i met something. It was boring, to say the least.