Especially if you want to quickly get rid of this awfully weak pawn on e6 with an e5 break, all White setups with b3 + Bb2 (and believe me, a lot of players of all levels like to play this against any Dutch) are quite unpleasant for black.
Black cant have his e5-break because of the threat Nb5 (Bb2xe5), but White easily gets his e4.
Do yourself something good and improve in the Leningrad, but dont play the I.Zhenevsky.

The Iljin-Zhenevesky is much more uncomfortable for Black than the Leningrad. Perhaps you could post a game or a variation that concerns you in the Leningrad and then I could give you a more specific answer about your Qb3 or weakened kingside problems. 

Keep in mind that if White aims for a quick O-O-O or an early h4-h5 without castling, Black can often mount a queenside pawn storm or castle queenside himself. (something you usually can't do in the IZ. Overall, I find the Leningrad setup to actually be safer, more flexible, and more dynamic than the Classical Dutch. However, as with any ultra-sharp opening, you are going to lose at least 50% of your first few games before you get a good feel for the opening. Once that happens, you'll find yourself winning a very high percentage of your games with the opening. In addition, the Dutch is one of those openings where you really need a good understanding of themes and strategies, so if you do not yet have a book on the Leningrad, I would strongly recommend getting one. Since you are from Austria, I presume you can read/understand German and so I would strongly recommend Steffan Kindermann's "Leningrader System". I do not know any German and was able to gain a great amount of knowledge from the book, so I can only imagine that if you are able to read and understand most of his prose it will be of even more help to you.

My dear chess friend, believe me, if you change the Leningrad to the classical Dutch, its like avoiding a small faux-pas (the higher KS vulnerability) for making an ever bigger faux-pas (weak e6, weak center, f-pawn missing for KS-attack).

@rousseau
As far as I remember - I tried the IZ only some weeks - Black has some means in the IZ too, for example after 8.Qc2 Qh5, 9.e4 e5 and no bad position for black.
Another way: 8.Re1 Ne4 and the knight blocks the white e2-e4.

But basically you might be right that the weakness of e6 could be a major minus of the IZ in some variations.

Well, there is of course a reason for the fact that the Leningrad is 10 times more often played then the classical Dutch.

In the Leningrad black manages often to come out with the e7-e5 breakthrough before White manages e4. But in the classical Dutch usually White is the first to break through in the center with e4 and Black gets then a by far not optimal position with the hanging weak pawn on e6.

I've played the Leningrad for more than a decade now, and can't say that I'm dissatisfied

With the Leningrad you should be prepared for both series of losses and series of wins, as one's current form is highly important. As well as theoretical preparedness of course, if you have problems against a certain white set-up you really need to do some work, both looking up theory and seeing what you do wrong and why in your games.

I've begun trying out the Iljin too, to get some diversity and fresh ideas, though so far only in blitz. But just like the Leningrad it takes a long time to understand and get used to the Iljin set-up and the various plans & tactics.

Just choosing a main line is somewhat perplexing, as 7...a5 and 7...Qe8 often seem to transpose after b3, my preference so far is the latter as black can play Nbd7 with quicker development - and the point of a7-a5 after b2-b3 eludes me somewhat. Why stop b4 if white obviously doesn't want to play it? Presumably, it's a waiting move, wanting white to commit with Qc2 or Bb2/Ba3 --- there is one  independent plan after a5, i.e. Na6, c6, nc7, Rb8 and b5 - fighting for square d5 - but I'm not sure when/if this is preferable to Qe8-h5/g6...

The books Williams & Pinski seem to favour 7...Ne4 but after 8.Qc2 Nxc3 9.Qxc3 I think white should have an edge - why move Nf6 twice only to give white Qd1-c2-c3, even if black can force through e6-e5 black's position just seems a bit dry to me...

Sorry to be adding to your confusion but maybe Iljin aficionado MNb can shed some light on these matters?