I prefer the silver. When I first looked at it I thought it was too pale but the more I looked at it the more I liked it. For me, and it's all down to personal choice, the grey is too dark (if it'd been iPhone 6/6S space grey I'd have chosen it).
I went for the aluminium in part because I dodn't want to spend more on a first generation product and in part because I thought the stainless steel was too corporate looking for my tastes. If I got another watch, I'd be tempted by the stainless steel to give myself the choice. The Series 2 stainless steel only comes with straps I either have already (white sport, Milsanese loop) or don't want (leather ones) so I'm not tempted.