Hin,
In addition to the 21 + 28-75 combination, I've also thought about a 21 + 28-105/3.2-4.5 combination, as well, mainly because the 28-105 is half the weight of the 28-75.
Granted, besides the great IQ of the primes, one of the reasons I want to have a nice prime kit is for when I want to travel light, so based on that reasoning, what would be the point of getting the 28-105?
So, at the moment, I'm envisioning my kit looking like this:
Lightweight prime kit: 21, 43 (or DA35), and 70. If I don't get the DA35, then I would add a DFA 100 for macro.
Zooms: currently Sigma 17-70 + Sigma 100-300 UC. Especially if I don't get the DFA100, then the 100-300 could also go with the primes for whenever I need telephoto.
I might consider replacing the Sigma 17-70 with the DA17-70, if it's weathersealed, otherwise I'll eventually trade the Sigma for the 28-75. Depending on reviews and finances, I would also consider replacing the 100-300 with the DA55-300.
With some of these new lenses not even being out yet, it's hard to make any definite decisions, but I'm keeping my options open.
Heather