Best UWA for NEX-6: Sony 10-18 vs Sigma 8-16 w/adapter?

Good morning -

I got a NEX-6 about a month ago that I am really enjoying.  I have been shooting it primarily with  two Alpha mount lens via the adapter (Sony 16-50 f2.8 and Sigma 50-150 f2.8) as well as an old Canon FD 50mm 1.4.  I'm very happy with my kit overall, but I know eventually I'm going to want to try something more wide angle.

Since my preference is for a zoom, I'm considering the following two lenses.  After reading a bunch of reviews however I'm not sure there is a clear "quality" winner - both have some distortion and other minor issues:

1)  Sigma 8-16

+ I already own the adapter to use this, it's the widest option, can get new for $600 or less used

- Size/weight

2)  Sony 10-18

+ Small size/weight for the NEX system, threaded so I could use an ND filter if I needed to

- Cost

I think if cost was equal I'd just buy the Sony 10-18 since it's optimized for NEX and hasn't exhibited any of the color fringing issues on the NEX-6.  But - $850 is a big chunk of change.  Do I just buy the Sigma and put up with the extra size/weight?

I also considered the Sigma 10-20 lenses - they are just a little less costly than the 8-16 but size wise they are only a little smaller, still maybe an inch bigger than the Sony.  So I wasn't sure there was a real benefit there.  I could also get a Tokina 11-16 f2.8 in Alpha mount and use with my adapter, but I might rather have a little more useable range and that is a big/heavy lens.

If anyone has real world experience with a bunch of these on a 16mp NEX body please let me know what you think!

Best,

- Mark