While waiting for MNb, I could offer my 2 cents: Ooops, too late!
The main line of 7...a5 and 7...Ne4 usually transposes early on, the first offers some more dynamic options and the latter reduces white's options a bit. All three books seem to like ...Ne4! btw.
Personally, I think the Leningrad is much more logical and easy to play, while my experiments with the Iljin have been both wine and water. In blitz I think I have a plus score but usually mostly because I'm good at taking my tactical chances in messy and bad positions...
Among other things McDonald emphasis that in the classical black should always be open to a transposition to an improved Stonewall.
While Williams' main rule is that black is OK if he can get in e7-e5.
Pinski concentrates imo on commenting whole games which makes it hard to get an overview...
So my advice would be to get the McDonald book first, as that would still leave you open to the Leningrad and Stonewall formations, if his commentaries should light a candle for y'a!
Anyway, as you've probably noticed, I'm biased in favour of the Leningrad as I've been playing it for decades by now. Still, after a couple of years trying to master the Iljin I still feel a bit uncomfortable around moves 7-12, as I'm constantly wanting to play Nb8-c6/d7/a6 even though I know Qe8 or Bf6 or a5 etc *should* be played... But if this syndrome doesn't seem familiar, then go straight to Williams' book.