[Card choice] What should I add?

[Team-Hero]

I have two extra slots opened in my deck and I am having trouble choosing a card to add. I would like to put in 2 Stifles because that would free up two extra slots in the sideboard, but how optimal are 2 Stifles in the mainboard? Are they worth it?

I am also debating between Ponder and Daze. Although they don't free up any sideboard slots, they are also good choices. Any other cards/ideas you guys may have?

Here's the current deck I am working on. I play it as a casual deck, kinda like my Sphinx deck. I don't expect it to be the best or most solid deck out there, but I like how it plays. (PS: I play VERY random decks)

4 Polluted Delta
2 Bloodstained Mire
7 Island
2 Mountain
2 Swamp
1 Badlands
1 Underground Sea
1 Volcanic Island
1 Volrath's Stronghold

Lands - 21

3 Jhoira of the Ghitu
2 Detritivore
3 Deep-Sea Kraken
4 Shadowmage Infiltrator
3 Dimir Cutpurse

Creatures - 15

1 Becon of Tomorrow (could be changed for another blue card)
3 Counterspell
4 Brainstorm
1 Beacon of Destruction
4 Diabolic Edict
4 Force of Will
3 Fact or Fiction
2 Ancestral Knowledge (I want to change to another optimal blue card)

Spells - 22

2 Umezawa's Jitte

Artifacts - 2

TOTAL - 60

4 Stifle
3 Planar Void
4 Extirpate
2 Pyroclasm
2 Ruination

[Aggro_zombies]

You know what Deep-Sea Kraken is awesome with?

Sway of the Stars.

[Team-Hero]

Oh that is cool. I can suspend that then right before it hits, suspend a Kraken! Nice. Now I need to get me one or two of these.