I think the dark matte looks best of the 2, but if I had a choice I'd try to pick a matte thats one of the colours in the image, say one of the colours in the clouds. The black matte is very bold and distracts a bit from the image IMHO.
Looking at my panoramas that my brother printed and framed for me, he used double mattes. The inner only reveals about 1/4" of colour around the image but it picks up the dark green of the foliage (they are all landscapes

) and the main matte is a cream colour (not so stark as white, obviously).
The width/thickness of the frame is just as important as the matte IMO and these look a bit narrow to me. They always say a good frame makes the picture...
The mattes on my panoramas are all >50mm from image to frame. Same distance all round looks good or a bit longer on the long dimension but never less on the long dimension

. For a pano 1000mm long, my frames are 1250 long I think with about 70mm matte each (frames in pairs in the same frame) end and the rest frame. My only solo panorama has the same matte width all the way round (50mm).
Al.