start with King's Quest 6

Space Quest games sometimes have references or jokes which non-native English speaker may not understand

That didn't stop me as a kid. :)
I started with SQ2+3, and fell in love with their approach.
Though playing chronologically does make sense, the first games in both series may be irritating, SQ1 is less of a pain in my opinion.

I think that King's Quest is more serious while Space Quest is more filled with humour.

King's Quests 1-2 are atrocious. KQ3 is merely bad. Space Quests 1-2 are dreadful.

I haven't progressed further yet. But I definitely would not recommend the early instalments of either series. They have not aged well. At all.

I started playing Kings Quest at V, went back and played the others I love the series (although the earlier ones take some adjusting due to their age as bazilisek said). I have never played Space Quest though so my recommendation is a little one sided.

This. I think it really boils down to which theme you want to play: fantasy or space Both are pretty comparable to each other and I don't think I could recommend one over the other.

Plus, as retro_gamer said, KQ is more serious and SQ goes for the laughs.

If you are going to play the early entries in either series, then do it with a walkthrough. They are filled with dead ends and stupid deaths.

KQ is historically a bit more important than SQ, so if you play them for historical reasons, then start with KQ.

Actually, I maintain the best way to experience these games, if you really want to, are longplays such as this one:
http://www.youtube.com/watch?v=EdNGpUjbPPA
http://www.youtube.com/watch?v=AtTkZ1_Tu3A

All of the terrible design in just 15 minutes, inevitable frustration not included.

Agreed. The early installments really aren't worth playing anymore.

Bonus: An actual quote from the comments on that youtube video: "games in the old days taught kids 2 read games today teach them how to shoot guns."

I'm inclined to agree, even with the excellent fan-remakes of the original games, I still would recommend checking out let's play videos rather than actually play them.

That is actually true for me. I learnt how to read due to Shadowgate.

So you are suggesting me to start both series with 4 + 5 + 6 ?

Get Space Quest 4, 5, 6, or listen to the humor on YT.

I spent the early years of my life playing Sierra *Quest games. I always liked the Space Quest series the best, but that is probably just my SciFi vs Fantasy bias. Like others have said its more of where you want the setting. If you want to travel bavk to the times of sword and sorcery then go for Kings Quest, if you want to visit a future with space travel, aliens and killer robots then go for Space Quest.

If you really can't decide you can split the difference and play Police Quest instead.

/nostalgia glasses: ON

Ha! Wimps! I played through the early Kings Quest games without the aid of a safety net - or walkthrough.

"Stupid deaths" well (teeny spoliers ahead):
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- if you put a foot or hand wrong whilst climbing a cliff, you die
- if you pull a rock larger than yourself down on top of you, you die
- mix a magical potion wrong and drink it, you die

"Oh dear, you died, we'll just resurrect you with everything you had right where you were, no harm done, would like like fries with that?" Not. Your dead.

At least the puzzles actually made sense within the world to which they belonged.