Well they do tend to grey out sometimes. As for which one will retain colour better, heaven knows if the lighter one will stay more purple or not. I will say that the second one already does have quite a bit of grey on her already, so I'd go for the first (but I'm also a sucker for dorsal stripes, so...)

In the end, it's up to you. If you want to go for more intense colours and pattern, get the second. If you want more muted, go for the first.