I can see how you would hold off for the thin booklet if you really just want the rules,
if it's rules that you are all after, then waiting for the mini rule book means you are still going to lose out. the large rules book has new rules in it for City Fight, Planet Strike in the form of special missions, that will not appear in the mini rule book. i too was going to hold out for the mini rule book, but with the specials not being included, to me it now makes it worth it buying the hardbound book.
I read through the giant rulebook, which i bought, and it has maybe a half page on each 40k expansion and then it tells you to buy the expansion, (apocalypse, cities of death, etc..) So if you get the small rulebook youre not missing out on any 40 expansion info or anything. Just artwork and photos, etc