TJ is correct.
I dont have any problems with a5 in the Leningrad (my personal choice up to now) or KID for that matter.

But the logic behind a5 in the IZ is not clear to me...

I think you refer to the Leningrad. I think a5 serves a similar purpose as in the KI. To avoid b4 and to put the knight on c5.

In many posts above we can read, that in the IZ double fianchetto Black has difficulties forcing e6-e5. That is not entirely true. One of the many possible ideas to meet this critical variation is favoured by specialist Keith Hayward:

Hahn,D - Hayward,K [A99]
corr 9USCCC, 1992
1.d4 f5 2.Pf3 Pf6 3.g3 e6 4.Lg2 Le7 5.0-0 0-0 6.c4 d6 7.Pc3 De8 8.b3 Pc6 9.d5 Pd8 10.Pb5 Pxd5 11.Pxd6 Lxd6 12.cxd5 e5 13.e4 Pf7 14.exf5 Lxf5 15.Ph4 Ld7 16.Le4 g6 17.Dc2 Ph8 18.Lh6 Tf7 19.Pf3 Lf5 20.Tfe1 Dd7 21.Ld3 Te8 22.Pg5 Lxd3 23.Dxd3 Tf5 24.Pe4 Pf7 25.g4 Pxh6 26.gxf5 Dxf5 27.Te3 Tf8 28.h3 Dh5 29.De2 Dh4 30.Tc3 Tf4 31.Pxd6 cxd6 32.Tac1 Pf7 33.Tc8+ Kg7 34.De3 Pg5 35.T1c7+ Kh6 36.Th8 Dh5 37.Kg2 Dd1 38.f3 Dxd5 39.h4 Pf7 40.Tb8 Kh5 41.Tc4 Txc4 42.bxc4 Dxc4 43.Txb7 Dxa2+ 44.Kg3 g5 45.De4 gxh4+ 46.Dxh4+ Kg6 47.De4+ Kg7 48.Df5 a5 49.Dg5+ 1-0

Now I do not claim easy equality for Black - such a thing does not exist in the IZ. At the 12th move though, Black has his ideal centre. It is not hard, to improve Black's play at move 16. This is not an exemplary game, but it strikes me that even with that awful knight on h8 Hayward managed to create counterplay.
For more games with this variation I refer to
http://www.geocities.com/drawyah

In the game Ahlander-Brynell I do not quite see where Black could have played dxc4 followed by e6-e5.
But is't 15...Bg6 (instead of Bh5) better? The pseudo sac Nxd4 did not work before, but after 16.a4 Qe7 17.b5 Nxd4 looks playable; I am not sure abut axb5 18.cxb5 Nxd4 19.Nxd4 e5 20.Qxc7. With the bishop on h5 the pseudo sac does not look completely satisfactory because of Nd4-f5.
I find 21...Qd8 rather passive; what about 21...Qc5!? It seems to be Black's last chance for counterplay.
From the Johnson-Lindberg game I do not understand very much, except the last moves 40.Bh3?? Rxd6 0-1. This indicates that for me the IZ indeed is easier to grasp.
For those who are interested: there is also a game with the Elephant Gambit.

By a strange coincidence, last nights' games in a local IM tournament had both an Iljin and a Leningrad, which turned out to favour the latter this time.

http://www.schackforum.se/Ahlander1.htm

Quote:


This I don't agree with! In my experience, black can get an awful position pretty quickly in either system if one doesn't play the toughest moves early on. If any of them is easier to learn, I'd say it's the Leningrad as the fianchetto is a more common set-up, which means you'll have some use of earlier adventures in the Modern/Grünfeld/ModernBenoni/King's Indian, which most people have dabbled with at one time or another.

I'm not sure what Brynell did wrong against Ahlander - my first reaction was to play for dxc4 and then e6-e5, but I haven't checked if this works concretely. 
If it really is that simple, why would Brynell play Bd7-e8-h5 instead, too drawish perhaps?  ???

Thanks very much for this, AmateurDragoneer! -- it told me everything I wanted to know.

Bjarnason--Goodger was wild stuff! ...

The original topic was to compare the IZ with the Leningrad. Just a few points.
Starting point: 1.d4 f5 2.g3 Nf6 3.Bg2 d6 4.Nf3.
I. After 4...g6 5.o-o Bg7 6.c4 o-o 7.Nc3 Qe8 8.b3 Black can play e5 immediately, while 4...e6 5.o-o Be7 6.c4 o-o 7.Nc3 Qe8 8.b3 it is harder to realize e6-e5.
II After 4...g6 5.o-o Bg7 6.c4 o-o it does not seem a good idea to play 7.b4, while this is dangerous against the IZ.
III After 4...g6 5.o-o Bg7 6.b3 o-o 7.Bb2 Black has not done too well, neither after e5, nor after Nc6, nor after Qe8; the latter scores relatively best with 43% for Black. After 4...e6 5.o-o Be7 6.b3 o-o 7.Bb2 Black can play for an advantage both with Nc6 and Qe8. Also Ne4 and a5 are possible.
IV I do not know the current status of the Karlsbad Variation. Against the IZ the Nh3 variations are not particularly good. Black can get easy equality with e6-e5 at once, permitting the exchange of the queens, while 7...Qe8 is the ambitious reply.
V playing Ne4 in the Leningrad does not seem to be a good idea in general; it is an important resource in the IZ.
VI The queens fianchetto hardly ever occurs in the Leningrad, while the setup b6, e6, Bb7, Be7 is rather popular, both with the d-pawn on d7 and on d6. This formation comes especially into consideration if White plays some quiet, unusual line.
VII In the Leningrad Black often plays c7-c5 or c7-c6-c5 with a White pawn on d5. In the IZ c7-c5 is usually not a good move.
VII In the Leningrad the pawn formation Wh c4, e4 has done considerably worse than in the IZ, with a White king's fianchetto. The reason might be, that in the Leningrad Black does not have the follow up Qe8-h5.
All in all I do not share the opinion, that the Leningrad is more flexible than the IZ. Due to the double fianchetto though, the IZ is more risky.

H'm ... interesting. But I'd still like to know what you guys / Kindermann think is wrong with 7 ...Qe8 8 d5 Na6 9 Rb1 (best??) Nc5, and what you think might be Black's best set-up(s) after 8 d5 a5 9 Rb1 Na6 10 a3 ...