Should I kill parthunax and help the blades? Or let him live and side with the Greybeards? Which group gives the most benefits?

I go Greybeards. You can complete the Blades quest before the quest to kill Parthuunax, and all they give you is a glitched potion. The armor and swords you can get earlier, and all they do is point you to Word Walls on the Open World.

Greybeards will point you to any of the word walls, and Parthuunax can strengthen one of your shouts.

If you want to read all the arguments, over and over ad infinitum ad nauseam, just do a search. I bet there are over a hundred threads about this just in the last month.

I have no idea of the rewards, but I chose the Greybeards, also, Parthunax is awesome, I can0't kill him

The Greybeards don't give you a ridiculous ultimatum like the Blades do. I think the answer is pretty obvious...

Go with the Greybeards. Delphine is a no good rotten [censored] that doesn't respect the dragonborn. Paarthurnax is hell of a lot better than both of these failed clowns.

Is there any benefit to joining the blades at all? Or are they just a waste of time?

http://uesp.net/wiki/Skyrim:Blades

It really isn't worth it to kill him, and Delphine is a cow, so I wouldn't bother.

The blades can svck my salty chocolate balls. Theirs no way that I would kill Paarthurnax just because some cow tells me to do it.

Watching http://www.youtube.com/watch?v=icp_OZTWH24 gives me such a mans pleasure. I think I'm going to spend some time with the Dark brotherhood.

Blades do give you a dragonslayer blessing and a unique enchanted sword, though.

Hands down the Blades. The Greybeards don't care about anything but their studies. The Blades are the one group the Thalmor actually fear and the Thalmor is the real threat after Alduin has been defeated.

The Thalmor don't give a [censored] about two old ducks quacking in an ancient temple. Just saying.

well, the greybeards gave u a couple of shouts and will direct u to a word wall, and alduin wil give ya a choice between 3 perks
Blades found u an awesome hideout, lets u recruit 3 followers and boost thier stats and keep them in 1 place, u get a perk, u get a temporary perk, armor, and unique weapons, and direct you to where dragons are.

Shout anywhere wheres theres people and u get a letter directing u to a word wall.
talk to innkeeper in riften and u get a dragon location.

So boils down to
greybeards--2 shouts, and paar gives ya a perk
Blades--hideout, armor, weapons, perk, temporary perk, raises stats of followers that are recruited

So ya get more from blades but ya gotta kill a dragon with a cool voice.....u cna do it, it aint that hard, trust me , u never know u might find out later u saved humanity..or doomed it...

Half Blade, half Greybeard, BLADEBEARD.

Nah I've gone with the Greybeards on two of my three characters. Blades have insanely cool armor, but the Greybeards are a lot more mellow and that dragon fellow up there is alright.

How about you simply do what you like instead of trying to get 'the best results'?

I highly doubt that the Thalmor fears 2 rejects that calls themselves the blades. I think they have much more to worry, like the next incoming war, rather then to focus on these 2 bums.

Most of the Blades "benefits" you can get before they order you to kill Paarthurnax. You can recruit 3 members into the Blades (Which you can still order to follow you around as a follower) and then get the bugged "Esbern's Potion". You can still take the "Dragonbane" Katana from the Temple. The only benefit that you won't be able to keep is the "Dragonslayer's Blessing" that Esbern gives you (Which is crap anyways, with how Criticals only take into consideration the weapon's Base damage).
With Paarthurnax still alive, you can switch your Word Focus at any time (Although the only one worth a damn is the "Fus" Meditation).

You can still get a shout-perk. And the Greybeards stop giving you locations after 20 words. While the random courier ones are infinite. (At least, until you get everything)

The blades offer some extra quests and you can take followers to them and power them up a little.

The offerings from the blades aren't all that great . If you have 100 in smithing you can smith yourself some badass dragonbone gear along with daedric weapons. Hell of a lot stronger then the blades armor and weapons. If you have a pretty decent enchantment you can enhance your weapons to sustain even more damage.

Oh and you forgot something. Once you tell the greybeards that you won't kill Paarthurnax and finish with the Alduin saga you can start hanging out at High Hrothgar.

Oh and the hideout rightfully belongs to the dragonborn, and not the blades.

Now I want to see a Nord with swords for a beard.

Now that's just crazy talk.

I killed Parth, I felt kind of bad, but I made him into a suit of armor so in a way he's always with me.

I quite like the Blades, so I often go either way. It's quite leet to be a Dragon Hunter.

Murder murder! MURDER!

Someone said that You can finish Recruiting Blades even w/o killing Paarthurnax. I just tried it and I'm stuck with yet another misc quest to ask Esbern about Dragon hunting. Problem is, he doesn 't have anything to say to me about it.

So how is it?

And about the bonuses either from Blades or Greybeards. At high levels, especially when You have Enchanting and/or Blacksmithing maxed out, they are all unnecessary.

That brings us to a question, who do You like more. Blades that want to protect all Tamriel from Dragons or some old funky dudes at the top of the mountain who don't care if the end is coming.

Oh yeah I still can't decide after months now...