I honestly don't see much resemblance of Chespin and Oshawott. I always disliked Oshawotts design, but I loved Chespin from the get-go.
http://25.media.tumblr.com/8a187e1a38ee1333235db5715bd1dd55/tumblr_mgcj7caEtr1qg2ts4o1_r2_500.png different head shape, body shape, eyes, nose, mouth. Seriously the only thing that's the same is the fact that they're STANDING. (sorry, couldn't find a better picture then that off hand)
As for these being the best designed starters? No, but they are surprisingly good. I haven't liked the starters since ruby, and these guys really are a surprise.
My favorite being Chespin, it is just so freaking cute, my second being Froakie, and third being Finniken. At first I liked Finniken the most, but the more I looked at her, the less appealing she was. And I also hated Froskie at first, but the more I looked the more I liked.
As to who I would pick, I really don't know. The last two games I picked the grass type, simply because I liked the design the best. So I want to try something different. But it all comes down to the other pokemon in the game, if there are a lot of fire and water pokemon I'll go with Chespin.