Hi Yipikaye, good questions.
I have the DA 20-40, and also the Limited Primes DA15, DA21, DA40 and DA70. I have previously owned the DA35 Macro.
The big advantage of the DA20-40 is it is WR and therefore makes for a very good quality wide travel walk around lens, suitable for all weather. I noted this in a review of the lens here on
PF.
Since purchasing the DA20-40 I have only had the DA21 on my camera a couple of times and the DA40 not at all. A good sharp copy of the DA20-40 is a very good lens, and genuinely is one of those pocketful of prime lenses. Sean Reid of Reid Reviews (subscription site) compares the DA20-40 favourably with the Leica Tri-Elmar. They have similar functions.
The DA70 is a very nice lens - small, light, and good for portraits and short telephoto uses.
I have found travelling that the DA20-40, DA55-300 WR and DA70 make a very good three lens kit that covers pretty much everything needed. If you need WR for a portrait/short telephoto lens then the DA*55/1.4 is another option.
In reality - all of the lenses you have mentioned above are good - so you can't get it wrong - it is just choices
Cheers
Ross