having owned a 10-20 for a long while and not having one now...I say go 10-20, even better look at the new 8-16 then add something in the 21mm area...I found I shot most of my shots with the 10-20 at 10mm and the rest around 18-20mm which means I would likely be happy with the 8-16mm and a DA 21mm over just the 10-20...I think I would get a ton of use out of the 8-16mm and the price is not that much more than the 10-20...
BTW, as I recall the 8-16 does include an adapter to allow using filters for 10mm and above w/o vignetting or very little considering...just a thought anyway.
EDIT: oops! I am mistaken, I would have sworn there was a filter adapter tube kinda thing for this lense but apparently there is no such animal...sorry about that. Still the images I have seen from this lense are stellar so far...unless the $120 or so difference is a deal breaker it likely should be part of the conversation.